I don't like Astarion, he's one of the only companions I've had in a game that I genuinely disliked. But I actually will say that to what degree he's truly evil isn't quite simple, and I don't think turning him good would be any sort of coppout really (if you ignore the 'undead are inherently evil' thing that I assume Larian are ignoring). Specifically because of this; when we meet him in game, (assuming we haven't been spamming rest of course) then he's been capable of choice and self determination for... a couple of hours at best. For the first time in two centuries. He's literally been completely under the will of a sadistic monster for two hundred years, physically unable to go against that will. I would argue that at this point he really CAN'T be evil because he's never had the capacity to make evil or good choices himself. Even with the choices he makes now, I do kind of feel (with some hypocrisy since as I said, I really don't like him) that he should on a moral and intellectual level, get a bit of grace for them. If I knew that a person had just gotten out of a years-long abusive relationship yesterday then I wouldn't judge them too harshly for their negative actions. That's the situation Astarion is in but greater by several orders of magnitude.
I doubt Larian is really going to capitalize on this nuance, but on an intellectual and philosophical level, that's why I wouldn't think it outrageous if he got a redemption arc. Because arguably, he hasn't personally done anything for which he needs to be redeemed.
