>They drink blood because their own blood accumulates factors that accelerate aging, and they need to periodically dilute it. Feeding isn’t nutrition. It’s dialysis.
This seems to be the emerging consensus. When you get older your metabolism creates all kinds of crap that circulates in the blood.
You would like to have boosted kidneys parallel to real ones that can detect and remove all the slightly wrong proteins.
Are there any reasons for this to work on non-vampires? :D
To reframe the argument, it's more likely that mechanisms for clearing cellular debris become less effective with age.