They ARE resistant, but they, too, have fallen FAR from where they were way back then. They're not capable of repairing their spawning pools, so they cannot expand that far because they always depend on that if they lose guys (which they will, because the Chaos Wastes quite literally EAT them.)
Also, while it's true that the background Chaos "radiation" was reduced since then, the times of glory where stuff on a scale like this (eradicating Chaos dwarfs, the Skaven etc) was even REMOTELY possible are past, as well. I mean, looking through the lore, you always hear about the ridiculously awesome heroes of the PAST, and the current ones are mostly seen as "the best X since Y" , meaning they still fall behind in some way. And even THOSE guys merely stalled the horrors and managed a makeshift solution for that moment. probably thinking "Better that than oblivion".
Ultimately, your question is something that we have argued about a LOT of times, internally as well as on this forum, and ultimately our verdict was that even if that was in ANY way possible, the time to be able to do so has gone by and at the point that our mod starts, the best case would be Vlad/Nagash killing everyone (perhaps but the lizzies), making the world so boring Chaos leaves it to him/them. Other than that, nothing seems quite realistic. Lizardmen are lacking in unity, Slann (especially Slann, since those can't regrow) and seem too stagnant in their defending the Slann and holy sites until the aforementioned lazer-hypnotoads figure out what to do, which takes ages and mostly just screws over the OTHER good guys.
And yes, Tech is advancing in the Warhammer world, but it's slower than in our world (from all we can tell) and seems distinct from the way things went for us. While HYPOTHETICALLY genetic modification might be possible, the fact is that that is so ridiculously far in the future we could honestly not tell if it'd work. At that point, you might just as well search a 40k-forum and ask there.
Also, minor tidbit/nitpick, but STRICTLY SPEAKING, Orcs and Goblins aren't quite Chaotic forces. They just wanna scrap and their emotions, for some ungodly reason, don't really feed the Chaos Gods that well, they go to Mork and Gork instead...somehow.
Orcs are REALLY badly explained, but hey, they're just fun.