One on one, O'Malley would beat Clinton. He would have had most of Sanders left-wing support plus less opposition from the party establishment.
Basically Clinton is a terrible candidate who would lose one on one to most people. The only reason she's barely beating Bernie Sanders is because he's a shouty weirdo. Seriously, I love Bernie but if he could just learn to talk like a normal person, he would probably have a 20 point lead nationally and have won Iowa.
lulz
Sanders led Biden in a poll by 20+ points. Try again.
Clinton dropping out vs. Clinton never running are two different things. Without the polarizing figure of Hillary and himself as the only real opposition, Sanders would have never taken off.
He would probably beat Biden if Hillary went to jail tomorrow but he'd have a much harder time if Hillary never ran at all and he was competing against Biden and a lot more from the start.