It's funny. Their stories, when they were good, never worked like this. They were simple and to the point.
The Horde story in Warcraft 3 is essentially about the struggle of breaking old habits, trying to start fresh, and stumbling on the way. It's simple, clear, and reasonably meaningful. It doesn't really fuck anything up.
The Alliance story was fine too. A decent corruption story that you could actually follow. You see Arthas become increasingly angry and desperate and see him justify more and more as his sense of purpose shifts from saving his people to vengeance for them. As was telegraphed in the earliest of missions. The core idea is there. If you sacrifice everything to beat your foe, are you still fighting for anything?
The Scourge campaign continues this. We see the end result of an Arthas who no longer cares about his men, who no longer cares about saving anyone, who has sacrificed everything to win. In short, we see a soulless monster. That's just the logical end result of "victory at any cost".
The Night Elf campaign is a simple story about essentially starting the true Age of Mortals before Cataclysm tried to redo it, but worse. The Ancient defenders of Azeroth are mighty and proud, but aren't strong enough to defend the world on their own. So they give up their pride and immortality to join the other races in defense of the world. And together, they can win.
It's fucking simple. WoW makes the mistake of trying to repeat themes without understanding them. We didn't need Garrosh, because that story was told well enough with Grom. We didn't need Cata, because the Nelves did the Age of Mortals thing better. We didn't need shitty "lol crazy!" corruption stories to manufacture bossfights, when the Alliance story showed one properly.