In the ending there is a dialog line about going on adventure to search for a cure for him. So they added it in because having him be cured was apparently to much for the game scope (would be nice for DLC though).
But maybe as consolation - Clerics (and if you don't play one you have Shadowheart as possible companion) can do divine intervention - of course limited in the game due to gameplay reasons but it would be a way to ask regularly (I think once per week?) your god(dess) for a divine intervention to cure Astarion. Normally it is up to the DM but I think a cure isn't out of scope.
We also know a powerfull wizard, Elminster and Gale has connections and a lot of knowledge (or at least access to books/information) too. So finding a wish spell / cure otherwise is not completely out of the picture.
So it isn't hopeless, just not shown directly in game but the dialog line given already implies that they were thinking about it and likely wanted to give players a way to address it.
And regarding the life span - if he is cured then he will (I suppose) have his normal life span again and as elf live 700 - 1000 years I think? And as Vampire spawn longer if not killed before it. But players can live longer too based on race and class - e.g. monks get with level 15 timeless body and don't age anymore? And something similar exists for druid (I think level 18/19)... and possibly other classes or with other possibilities as well. It isn't in the game right now but it isn't hopeless.
But I understand wanting to have a happy ending with your love interest and ideally it should be shown in the game directly.
Otherwise I agree completely, the whole having him running away from the sun and others just standing there giving a dry comment was NOT OK. Both if romanced or not - it is bad enough that you don't get ending scenes with your companions if they aren't romanced (and some romanced ones aren't great either). We need an opportunity to be able to speak with all of them to say goodbyes or make plans etc. even with camp followers and allies.
I'm, personally, not the sort of person who tries to headcanon my way out of bad writing. I understand what you're saying, where you can kick back and imagine what your characters, or characters, would do after the game ends. But at the end of the day, for me, if it's not explicitly shown or stated, or implied or insinuated, it doesn't happen. At no point in the story-line does anyone express these sort of things as options, or even say, "Hey, maybe we can do this". So, at the end of the day, it just doesn't happen in my mind. Doubly so because all those characters are the primary ones hurling insults.
