There's no reason why Marat couldn't win Wimbledon. Maybe the only reason is Federer, but there's nobody else out there that you can point at and say "he's clearly more dangerous on grass than Safin." Like people have already said, it's mental. Lendl hated grass, too, but when he decided to really apply himself he became a great grass court player and he doesn't have half the talent Marat does.
BTW, I was watching the Aussie semi's again and Federer was really having an off day. Sure, Marat was playing good, but really it was all about Roger playing very poorly (at least what I saw, up to third set). It's scary to think that he still took Safin to a close fifth set.