He'd have lost like the rest of the Republican field. This year wasn't a Republican victory. It was a Trump victory. Any other Republican put up against the Clinton juggernaut would have folded quicker than your aunt at a poker game.
Yeah, the candidate with the highest unfavorables was clearly the most electable Republican out there. I get Trump's appeal in states like Michigan and all of that, but you're seriously underestimating the number of Republicans/suburbanites/Whites that only voted for him very reluctantly or not at all.
Also... Clinton "juggernaut"? Seriously? She basically destroyed him on the airwaves in what was the most negative campaign ever. He never responded to those devastating Democratic attack ads. If that didn't hurt him, it wasn't going to hurt Kasich either.
That's because she dumped so much dirt on him that he became teflon. She figured he was so unelectable that she'd never have to give her own vision. She wouldn't have had to run that kind of campaign against Kasich. She'd have beaten him on her own merits like Obama did over Romney.
Suburban Republican whites always vote Republican. They don't change the election's course. This election was won by Obama-Trump voters who wouldn't vote for Jeb!, Kasich, Cruz, or Lil Marco.