Honestly , I have zero issue with him being a rogue, and the OPs arguments are specious to be kind.
I *had* no problem with him being a vampire... until I read this thread. Those arguments are very sound. The suggestion of simply making him a vampire thrall is elegant and fixes a ton of issues.
Honestly my bigger issue was him being a vampire AND an Elf. It’s a hat on a hat. There comes a point when you have so many rare background elements that it strains all credulity. Just make him a human. The party needs more humans. Toril is a human dominated world. Not everyone is a super rare snowflake. Plus being an elf doesn’t play into his character one bit. It’s purely cosmetic. You could make him a human and not change a single line off dialogue.
You make a valid point. Him being an elf is truly just cosmetic, and it would in fact make way more sense for him to be a human if he really wants to stay alive so much. Humans live short lives, it would truly make more sense for one of them to be obsessed with survival.
However is you make him human you have to keep him a vampire-spawn. Thralls age and die, and Astarion served Cazzy for 200 years. Thralls are not actually undead and they must receive blood from the master vampire every week or they lose all their powers. Astarion is going to hang out with us for far longer than that.
So if you make him human he must stay a spawn.