I'm gonna say it: If someone came to me as a DM with Astarion as a PC, I'd say yeah. It's a pretty good character concept, and I've played with way weirder, and saying "sure yeah you can be a vampire pretty much in name if it helps with this cool subplot I'm down for that" seems like the more reasonable thing than saying "NO vampires HAVE TO BE challenge rating 5 and THEY CAN'T DO ANYTHING BUT EAT BABIES", wouldn't you think?
And you know what? That’s perfectly fine at your table. The problem is, even though the game is meant for each person, all the lore and companions affect everyone’s individual stories. Because we are all influenced by the story, it should try to stick with existing lore.
Each change that Larian makes doesn’t just affect one person. It affects everyone.