Would an altmer (high elf) have a reason to become a vampire or vampire lord, since according to lore, high elves live up to 1000+ years... ?

I would say only if he thought the powers involved were worth it. and of course, if he happens to be 995 years old... too bad the game doesn't really allow one to become a lich, eh?

There is a mod on nexus, that lets you become a lich, but from the trailer it looks a bit "clucky" but still good. The problem i have is that i'm a power hungry altmer wizard, and don't know if vampirism has that effect on altmer as it would be lets a nord or imerial. Now if i played a nord mage, then yes, because nords live 70 years give or take? but altmer? not soo sure. The only way i see if i was already infected and accepted the gift from harkon. So yeah...

Vampirism is more than eternal life. Keep that in mind.

Some Altmer see themselves as a superior race. Some Altmers don't care what those Altmers think, because they're already dead anyways.

You can roleplay that your character is dying from some incurable disease. But still has enough strength left to save the world or go questing. But eventually there time will come.