There are numerous ways to cure vampirism in DnD 5e and we have at least one at our disposal from the start - Withers, which is completely ignored.
Yep to that, plus how many other gods have we possibly done favours for throughout the game, Selune and Mystra are also possibilities for obtaining a wish spell. One of the three should be amenable to persuasion. Withers can certainly do it for Astarion and very easily considering he's right there, maybe after seeing Astarion choosing not to ascend he thinks he's suffered enough.
I don't think unascended Astarion (enslaved and tortured for 200 years, awesome redemption arc and gets punished for it) and Karlach (a total innocent) are treated the same way the other romance options are. I don't want poignant, I want the same chance at a happy end as other romance options have (dependant on my choices), I'm happy to work for it but the options aren't there.
