The legion just cannot properly move it's army around or field it's more powerful members efficiently.
In any realistic scenario, the zerg simply cannot be defeated. Even if, by some miracle of archimonde + kil'jaeden + sargeras nonsense that they managed to kill all of them on a single planet, there simply aren't enough members of the legion to leave on that planet to KEEP it secure and prevent another infestation. They would have to move all of their forces to take each planet and be unable to actually hold the planet once taken. They simply cannot win because there aren't enough of them, nevermind the fact they are drastically weaker than zerg physically and structurally.
New York City has 8 million people living in it. That is more than 4 times the population of the entire burning legion. And their quantity is canon. Imagine how small new york city is compared to the world at large, and imagine a military that tiny trying to maintain control over an entire planet. It simply couldn't be done.
I don't think you people grasp the numbers or the realities of this situation.
Let's consider population density for a moment. I'm not sure exact land mass measurements have been given for azeroth so i'll use the real world as an exmaple because those numbers actually exist.
With the legion's CANON population sitting at 2,000,000... That gives them a population density of 0.0345 demon per square mile. Think about that for a minute. Not quite 3 and a half fucking demon, every 100 square miles. Zerg population on the other hand is theoretically in the trillions but I won't even need to speculateor use common sense or guesswork, i'll simply just use hard, confirmed numbers given in the lore. Population of the planet Char is around 10 billion in the year 2504.
So if all of those zerg, JUST on that one fucking planet, moved to earth, their population density is going to be about 172.4 zerg per square mile.
SO stop and think about those numbers for a minute. Try to wrap your brains around that. For every 100 square miles on the earth, you' dhave 17,240 zerg tearing the fuck out of 3 and a half demons. I mean, are you people seriously even entertaining the possibility that the legion can win? Really? Even if the zerg didn't so grossly, overwhelmingly overpower them with being harder to kill and having more damaging attacks, they are just way, way, way too outnumbered. Even if only one tenth of a single percentage of the zerg were ultralisks, that still means over 17 ultralisks that can simply crush them to death without even having to attack vs not quite 3 and a half demons.
The numbers is where the zerg win so categorically. Even if you ignore the fact they are far more resilient and more damaging and threatening in combat to the point where one zergling would be stronger than most of the demons in burning legion, just the numbers alone have zerg winning.
And you might think of something dumb like "well they wouldnt be that spread out", but it doesn't matter because if they were concentrated in a single point, the zerg could surround and attack that point from all sides. Know anything about military tactics? You know how overwhelmingly fucked you are trying to defend against all sides at once? Even if you were to assume they were roughly equivalent in strength by ignoring all of the physics behind everything we know about both groups, and having it lean heavily in the legion's favor, it simply wouldn't matter. There's just too many zerg.
And again, sargeras can't stop them without blowing up every planet in the universe which is something he either can't or won't do. Any way you look at it, it's a no-win scenario for the legion. And they can respawn from now till the end of time, but those overwhelming odds against them will never change and in fact only get worse. Those zerg densities will increase over time and make it more and more in the swarm's favor.
You guys have to consider, the zerg were basically ripped off from the Tyranids from Warhammer 40k as all the starcraft factions were ripped straight from 40k and slightly modified. And in 40k, the armies are bigger and stronger than anything in WOW. ANd yes, they even fight against magic and even gods in that setting, and still win. Tyranids, like the zerg, just multiply too quickly and there's just too many of them. It's one of the 4 "dooms" of the 40k universe, one of the potential endings to the universe is that the Tyranids eat everything.
It's just not an enemy you can defeat sadly without some heavy plot device nonsense. You can fanboi about legion and pretend they're super strong all you want, but they routinely lose and have yet to succeed in their goals.
Meanwhile, zerg keep eating planets and the situation is so hopeless Protoss purge those planets rather than even trying to take them back. It's the root of most of the terran vs protoss war and tensions in the first place. Terrans want to try to evacuate people to save them from those completely fucked planets, protoss want to purge it before they have a chance to spread elsewhere and contaminate another planet.
Also Kerrigan became a god at the end of SC2 so... She could take Sargeras I think.
If the legion army was TRULY overwhelming or unstoppable, they would just invade and be done with it, but they don't. They know they are not that great and kinda flimsy, so KJ tries to soften up enemies first. Tries to make populations turn against themselves to cause strife and disruption and all kinds of other shit to make them easier to take over. If they were truly powerful and overwhelming, they wouldn't give a fuck. Zerg certainly don't. And you aren't going to have KJ popping up whispering sweet nothings to the hive mind of the zerg and convincing them that fucking over their own kind is a good thing to do. Isn't gonna happen. Legion just can't compete against something like the zerg, they have absolutely nothing in their favor in such a conflict. And seeing as how one orc actually hurt Sargeras that one time, he'd fall to the overwhelming swarm as well. He isn't invincible.