It's not like the Asurs are bathing themselves in the Vortex. And even if it's dangerous to be near it (which nothing even hints at), the Druchii are right next to the Choas Wastes, which would have an even worse effect.
I will note that "Asur" is both singular and multiple. "An Asur" refers to a single individual, and "The Asur" refers to them as a whole- you don't say "The Asurs".
This is the stated reason for the Eldar's low birth rate in 40k.
I was under the impression the limiting factor on Eldar population was the lack of spirit stones as well as needing to avoid attracting Slaanesh, and that absent those factors Eldar fertility is just fine.
I could have sworn it was specified somewhere, but a qick check of ET: Nagash doesn't come up with anything.
Aliathra's lore is super weird anyway, because they were very obviously and hamfistedly trying to set up the whole "Teclis wants to resurrect Nagash to fight Chaos but doesn't want him strong enough to take over, so he tricks Arkhan into using Aliathra as a sacrifice but because she's Tyrion's daughter and not Finubar's, Nagash gets the Curse of Khaine rather than the blessing of Asuryan" twist in the End Times.
But that doesn't work on multiple levels. First of all, the whole Khaine/Asuryan thing doesn't make much sense, Alarielle is a descendant of Aenarion same as Tyrion, and is the daughter of a Phoenix King, so Nagash should have gotten hit by the Curse of Khaine even if the plan worked perfectly, and blessed by Asuryan even with Teclis' sabotage. But it's whatever, that's End Times nonsense for you.
But far more annoying to me is that Aliathra's entire situation makes no sense. According to the armybook, the Everchild must be the firstborn daughter of the old Everqueen, sired during her year-long marriage to the Phoenix King. But Tyrion and Alarielle couldn't keep it in their pants so Aliathra is Tyrion's daughter, not Finubar's. But the problem comes from the fact that
Finubar is almost certainly Alarielle's father.
Tyrion was born six years before Finubar took the Phoenix Throne, and Alarielle is younger than him. Ergo Alarielle can't be the daughter of Bel-Hathor, because she would have needed to have been conceived when Bel-Hathor took the throne, and that would make her 650 years older than Tyrion. Even setting aside the explicit statement that Alarielle is younger than Tyrion, that is also very much how she's presented, young and inexperienced.
The only way Alarielle isn't Finubar's daughter is that the Everchild doesn't need to be the firstborn daughter and Bel-Hathor for some reason conceived her in the last few years of his life. But if the Everchild doesn't need to be the Everqueen's firstborn daughter,
then there's no need for Alarielle and Tyrion to hide Aliathra's parentage, because as per the armybook the Phoenix King and the Everqueen are both explicitly allowed to take on other consorts and have children with them, it's just that those children cannot become the Everchild. Finubar and Alarielle could just have another child.
So for the narrative around Aliathra to work, Alarielle needs to be Finubar's daughter, which means that as far as everyone else thought, Aliathra was the result of father-daughter incest. Which, first of all,
fucking ew, and second of all, we
know that the Asur have a taboo against incest because Yvraine cited it as a reason who Malekith can't become Phoenix King- he'd have to marry her and they can't do that because they're half-siblings.
Something has to give. Either Aliathra's backstory doesn't work, or the Asur are fine with incest. And let me tell you, at least as far as MMQ is concerned, it's not going to be the latter.