I think that curing or not Astarion vampirism is a choice. As many people said, the magic in D&D allows a vampire to be resurrected. It is not easy and it is expensive, but it is possible.
But the real reason that I say it is a choice is Minsc. If they have come up with a "reason" for Minsc, a human, to be still around 200 years after the original Baldurs happened and also have Sarevok, another human who might have died or not, they could also find reasons or ways to cure Astarion. Maybe they want that for a DLC or Baldurs 4, who knows? But in a fantasy game, it is possible. Also, they have bent the rules whenever it suits them. That is ok, but they have to own their choices.
