If you look at the story close up, it's a lot more realpolitik than if it had been a traditionally "well-told" fantasy story.
I mean, the war was literally won with an SI:7-backed coup LMAO.
Anduin took the figure of a graying, grizzled old general who opposed the current Horde leadership but at the time had no particular plans or allies -- he only saw suicide as an option. Instead, Anduin took him back to Stormwind Stockade, then released him on the condition that he form a dissident faction opposed to the Sylvanas regime. SI:7 agents abetted him, helping him travel safely.
To lend the dissident faction legitimacy, the first move was to go trot out the old retired founder figure of Thrall, not unlike how Prophet Zul tried to gain the appearance of legitimacy by resurrecting the dusty zombie of the founder-king Dazar. Thrall is a figure who, like Saurfang, articulates almost no discernible political positions, only a vague call to "restore honor to the Horde". He was, in fact, the person who first chose to hand over the reins to the military reactionism of Garrosh. But when Garrosh did exactly what he always said he'd do, Thrall acted surprised and backed Vol'jin's insurgency. I guess Thrall assumed that Garrosh was as cynical as he is, and used "blood and thunder" rhetoric only as an empty gesture to appeal to orcs who have nostalgia for the Old Horde. Or maybe he simply bowed down to the political reactionaries when he thought that was the "mood of the times", perhaps fearing that if he didn't appoint Garrosh, the Horde would fracture in two along political lines -- of course, it ended up doing so anyway, and Thrall's choice meant that Garrosh enjoyed the upper hand in the ensuing civil war. Finally and more banally, Thrall may have assumed that Garrosh would follow his lead and could be controlled, but if he did, that assumption had no basis.
Thrall's main takeaway from the Garrosh fiasco seems to have been that only his close clique of confidantes can ever be trusted to run things. As such, he is more than happy to put his thumb on the scale for his old buddy Saurfang. That this involves directly and illegally interfering in the line of succession, since Sylvanas was the handpicked successor of Vol'jin, clearly doesn't bother the old kingmaker. He is also happy to bring in his old buddy the corporate contractor Gazlowe to run the Bilgewater Cartel, despite having no legal authority to appoint their leadership. It becomes clear that he even trusts Jaina, another old buddy, more than most of the Horde, despite the fact that she's been out and about killing Horde left and right.
With Thrall's endorsement secured, Anduin then arms and gives military support to the dissident "movement" he created, or rather, fabricated based on the discontent of a single disaffected high-ranking military officer. They mount an armed coup.
The people performing this coup freely admit that they are not a populist or popular movement; according to their own words they are greatly outnumbered by Sylvanas's loyalists and armies, even with their numbers doubled by Alliance support. That's very different from Voljin's rebellion against Garrosh, which received widespread Horde support, with Garrosh's forces comprising only the core of orcish loyalists whom he hadn't expelled from Orgrimmar, plus some goblin mercenaries.
Also, while Vol'jin's rebellion did eventually work with the Alliance to topple Garrosh, the two forces were always separate, and the rebellion was always in Vol'jin's control -- the divide is seen all the way up to the MOP ending cutscene -- whereas Saurfang's rebellion was engendered by, fueled by, and is ultimately inextricable from the Alliance.
Saurfang is joined by Lor'themar, who feels the war is "draining the Horde's resources", and who had previously tried to get his people admitted into the Alliance, by Archmage Thalyssra, and by Baine Bloodhoof, who has notable Alliance sympathies -- he banished any tauren who fought back against Alliance soldiers invading tauren lands, and has kept a longtime personal correspondence with none other than Anduin Wrynn, who he considers a "friend", a sort of relation that no other Horde leader has found proper. Baine is arrested after he sabotages a Horde covert operation and illegally returns an important prisoner of war to the enemy, but he's broken out of prison by the other insurgents.
So what do you call this "rebellion" that comprises a small, unpopular group of politicians and military leaders, created, supported and bankrolled by the Alliance, coming together to oust a regime with which the Alliance is at war? A coup, obviously, but what are the motivations of the different actors?
Lor'themar and the blood elves have shown interest in belonging to both factions, depending on what was convenient at the time. A peace in which they get to trade freely and be on good terms with both factions is certainly to their advantage. Unlike the Forsaken, who will never be truly welcomed by the Alliance, the elves have no fundamental reason why they have to stick with the Horde and therefore don't much care if, as Sylvanas predicted, the Horde gets shafted in the long term by such a peace.
The nightborne didn't like re-entering the world just to immediately get thrust into a war, so they want that to end.
Baine, meanwhile, clearly does believe (and perhaps this vision was developed in his correspondence with Anduin) in a globalist, post-faction future with free trade and open borders. As we later see, he is right at home visiting Stormwind alongside Valeera, a neutral agent who does espionage for, and upon, both factions. With national ties to Silvermoon but personal loyalties to House Wrynn, Valeera is the kind of post-faction Davos Man who epitomizes the Baine-Anduin globalist dream.
As for Saurfang, he has no real forward vision and never has -- remember, he just wanted to suicide before Anduin put him up to this, and in Legion even his friend Eitrigg questioned his mental state. He's tired and confused, and his internal turmoil and lack of a clear vision for the future make him suggestible. Saurfang obviously feels a lot of guilt for the events of the First War, and he has always used "honor" as a way to feel cleansed of this guilt. In this, he is not actually escaping the mistakes of the past, because that's precisely one of the ways the orcish honor system functions -- giving you personal-scale behavioral taboos that let you exculpate yourself for participating in larger atrocities. For example, Saurfang had no issue with leading the invasion of the night elf lands, but when he refused to execute one person because they were attacked from behind, he gets to feel high and mighty, even though he was the general who led the invasion. That he was willing to commit treason to maintain this facade just goes to show how important it is to maintaining his psyche. Regardless, this guilt is what Anduin plays upon to manipulate him.
But in one way Saurfang has no illusions: talking to Anduin before the battle, he admits the hollowness of his and Thrall's "honor" rhetoric, declaring that the Old Horde never had any honor to begin with. And you have to admit, all the "honor" talk that he and Thrall substitute for a concrete agenda glorifies and whitewashes something that they are, in reality, opposed to. Of course, this rhetoric was important when Thrall was trying to unify the orcs to form the New Horde: it appealed to those who had a nostalgic view of the Old Horde (a demographic Thrall has always moderated his positions in order to court, see also his appointment of Garrosh), and it gave a traumatized and transplanted people a feeling that their past was good -- that old orcish society represented noble ideals. In a way it was a sort of doubletalk or litmus test, able to be heard either as an allusion to Old Horde militarism or as a call for rejecting it. Often it was heard, contradictorily, as both at once. The word honor as Thrall used it was like a compressed emulsion of the contradiction he had to grapple with to unite the orcs (though of course, the emulsion came apart during the Garrosh episode).
So that much Saurfang sees clearly. But by simply branding the Old Horde's atrocities as "not truly honorable", he refuses to face the fact that it IS the very honor system he holds dear that was complicit in those acts. The "honor" system acted to maintain a very specific social reality -- the warlike society of the orcs on Draenor. If you don't want that kind of society, you can't idolize "honor". To have a successful character arc, he would need to realize that the "honor" he clings to is piece and part of the things he feels guilty for. As a consequence, he would realize this "honorable death in battle" thing he has imposed on himself is not a real solution to his problems. But ultimately he isn't able to solve this contradiction within himself, and instead, by challenging Sylvanas to mak'gora, he achieves what he has always Freudianly desired, a theatrical spectacle where people have to watch his personal death-fantasy being fulfilled and validate it. (This is another example of how he is motivated internally by his feelings and psyche, not by any external political ideas or vision.) By a ridiculous deus ex machina that seems more like some wishful daydream of Saurfang himself than anything plausible, this ends up causing Sylvanas's supporters to all suddenly abandon her and embrace the coup as legitimate. That one's a headscratcher.
But the result is that while Varian Wrynn had to bash down the gates of Orgrimmar, the Horde welcomes Anduin in. All by using soft power, Anduin gets the Horde to install leadership favorable to the Alliance, run out of town those who are anti-Alliance, and permanently demilitarize (no more "Warchief"). He installs Calia Menethil to "advise" (oversee) the Forsaken, and a rebuilt Stromgarde promises to replace the Forsaken as the chief power in Lordaeron. Under the illusion of an equal-terms ceasefire, all while seeming nice and gracious, he has relegated the Horde to an inferior global power doomed to lose out economically to the Alliance, exactly as Sylvanas feared and foresaw in "A Good War".
And who opposes this treaty? Exactly the people who lost the most in the war, the night elves and undead. The treaty gives them nothing and no particular future. That's not the point of the treaty. The point of the treaty is the rich species telling the poor ones: forget your vendettas and your homes and ways of life that were destroyed, from now on it is all open borders and free trade. Maybe the Horde elite will get richer even as their faction as a whole grows geopolitically weaker, but the losers are the most disadvantaged people on both sides.
----
The character of Anduin is much more sophisticated than is recognized. He's an effective politician who uses his sweet and saintly manner to manipulate people and get his way while seeming unblemished. The crowning example of his canniness was his plan to defeat the Horde by creating the Saurfang coup. How can it be any more explicit how he used Saurfang, than that he literally enters Orgrimmar using Saurfang's corpse as a Trojan Horse? He walks through the enemy gates as a pallbearer for the dead hero. That's political brilliance. It doesn't make him any more likable, but the worst part is, I think he's completely sincere -- I'm not saying he's intentionally a cynical mastermind. The most dangerous manipulator is the heartfelt one.
Anduin also has a very satisfying story arc in BFA. Anduin's character struggle has always been the contrast between his softer, meeker nature and his great warrior father. BFA shows Anduin successfully resolve this struggle. Varian understood hard power and force, but Anduin understands soft power, and this understanding allows him to achieve a quieter, but ultimately much more effective victory against the Horde than his father's victory in MOP, which evaporated almost immediately with the rise of Garrosh 2.0. Learning from his father, Anduin realized Orgrimmar could only be taken if the Horde were split against itself, but by being intertwined with the rebellion from the start, he was able to control it in a way his father wasn't.
Conclusion: This story of the Alliance, the overall stronger faction, winning the war by instigating a coup within the underdog faction and ultimately convincing its elite leaders that peace would be more profitable to them, with the result that they oust a popular wartime leader and install globalist policies that ignore the disadvantaged, isn't an exciting fantasy story but it does seem unintentionally realistic, and does in fact end up being "shades of gray". It also shows us characters who are more complex than Blizzard itself notices.