I know people were asking why the Asur are dying off and not the Asrai/Druchii, and I think I can come up with a pretty good reason: they're too busy trying to keep the world turning with no back-up. Asur are constantly patrolling the oceans, sending armies to check on waystones, slapping uppity chaos incursions, and so on. They don't live in the giant wellspring of Ghyran that is Athel Loran, and they don't go out picking fights with helpless peasants like the Druchii do. They are consistently pulling off a patrol of an entire death world on both land and sea while fending off opportunistic kidney punches from their edgy cousins up north. That wears on them. A lot.
There really isn't a good reason. Ulthuan has the Everqueen and is pretty much never invaded due to being an island sea power. It also has a super fertile magic verdant land, a society actually suitable to raising children, etc.
Like, the Asrai should be the one dying off because they live in a magical death forest and constantly get into fights with the people around them, and actually are at risk of things like Beastmen, Chaos, Orks, etc.
But this misses the logic WHF runs under - The Asrai are assholes, and therefore have magical breeding powers. The Naggarothi are even more assholes and have offscreen magical portals of dark elves. The Asur are relatively nice, and therefore are emaciated and slowly dying.
Apply this logic further and you see how it functions - Chaos are evil, so infinite. Skaven are evil, so infinite. Undead are self destructive, so infinite. Once you understand the underlying logic, suddenly you realise that in order to survive in the world of warhammer you absolutely need to do acts of random evil, or at least random assholery in order to keep your population going, double so if you're on the 'good' side. Suddenly, the Asrai randomly attacking humans, or the Empire deciding to outlaw and genocide wizards, or the Brettonia dissapearing all guy magic users makes perfect sense - If they didn't, they would suffer a mysterious sickness on a population scale incurable by any in-universe method.