If he did step down Clinton would pretty much win because it's far too late for the Republicans to pull a comeback unless something devastating happens to Clinton too. If Trump steps down then the RNC can pick a new candidate.
The backlash to Trump's comment comes from all angles including his own allies and even his wife so it's probably the worst you could ever get. While Clinton has done some pretty shady things it has not gotten no where near as much backlash from the public and the media as this. Or at the very least, there was no major call for Clinton to quit her campaign while this is the second call for Trump to quit.