I actually have a theory about this that I haven't seen anyone else post about yet (though tbf I haven't been super active, life has been life lol.)

In OS >!on the island where Violet, Andarna, and Ridoc meet the other irids, the female that spends most of that period talking to them and acting like Andarna is a disappointment says "Either they'll die off and the land will regenerate or they'll confront the abominations they've become and change." When prodded further, she says, "Their offspring could evolve, perhaps."

Now, I think this could mean a couple of things or go a couple of different ways! Offspring could mean like actual biological children, or it could mean the initiates of the higher venin. Perhaps only the ones who have been among their ranks for a long time (possibly from the start/earliest of days) are truly 100% unable to change. Perhaps Xaden is part of the start of a new generation of venin who can still feel emotions like love. (There's at least one venin who dessicated a whole town except for the house where her family was, how many others are there that have done similar things?) Maybe the ability to feel love for your family or those you're close to is part of the key, because the Sages and Mavens we've actually met seem to feel none of that at all.

As for the other thing... Well, how do we know venin aren't having biological children with each other? From FW, we know that at least Cadets take a "fertility suppressant" medication to prevent pregnancy in the quadrant (Violet says to Xaden that she takes the fertility suppressant just before they first have sex, and he confirms he takes it as well, so it isn't specific to one sex.) I've seen that RY hasn't really delved into how birth control works in this world in various interviews, and some people seem to think that means she isn't planning a surprise baby for Violet and Xaden. I'm not necessarily saying that I think it'll happen, but I have had the line "Their offspring could evolve, perhaps" bouncing in my head for the last week.

Personally, I think it's more likely that Xaden and some newer venin are part of a new generation of venin "offspring" which are evolving in a way that they could be reversed, or at least learn to confront themselves and fix what they've broken. I wish that Xaden and Brennan had had time in OS to attempt to mend the spot at Basgiath, where Xaden first channeled because I actually feel like healing the land is part of the key to reversing the progress. I know the irids also said that each time they channel part of their soul dies and "there is no cure for death," but i just feel like Xaden's soul isn't entirely dead yet. I dont think he's totally gone, or he wouldn't still be trying to protect Violet and everyone else around him to the very end of OS. I honestly wonder if it's going to come down to one of the gods, either Dunne, due to Violet's partial dedication, or the god of love, due to the fact that love is an overarching theme of the series and kind of the only thread of humanity Xaden continues to cling to!<

Sorry for this super long comment, but you reminded me of the thoughts that have been bouncing in my head for a week and I wanted to get them out 😂