Some people would like that. Other series, including Max Payne, have remade their characters in HD. But Rockstar feels the graphics in pre-GTA IV games defined those characters as too cartoonish to exist in an HD interpretation of the world.
I'm not going to be harsh but next time use the search function, but I'm still happy to answer your question. I would rather have a new protagonist, Cj was a brilliant character to play as, but I wouldn't like to play the same character.
Most people want new protagonists and fresh storylines but are just nostalgic about past games. Simple as that, they hope to see some reminders of the old days. Like when your grandmammy bakes cookies for you in college and you suddenly feel like a kid again even though in reality youre a drunken adult.