The I Am Legend movie suffers from a couple of glaring flaws.
First, the story misses the point of the original novella. Our vampire-slaying protagonist does not become a legend to a colony of plucky survivors who owe him a debt of gratitude for his brave self-sacrifice. He becomes a legend — a boogie-man, really — to the perfectly intelligent non-zombie vampires he’s been killing in their sleep, as they go on to forge a new non-human civilization.
Second, if there’s one thing we know we can do with practical effects, rather than computer graphics, it’s undead ghouls. Special-effects artist Steve Johnson demonstrates what I Am Legend would have looked like with non-CG monsters: