In the states where Trump is winning, a large majority of those states, and maybe even all of them, are dark, dark red. The blue states where he enjoys some support but are mostly Democrat, they wouldn't carry him in the general election anyway, and with or without the support of Trump's voters in red states, they'll have enough support to stay red, as they usually always have, even with a boring, stale Republican candidate like Romney.
What makes things different for Kasich isn't that he's THAT different than Romney, but that he polls against Hillary Clinton very well because Hillary Clinton is a ty candidate (whereas Obama was not, at least according to polls, since he consistently beat Romney in them) and because the DNC has tried to rig the primary in her favor despite that fact. Several states Obama won by relatively small margins (eg Iowa, Colorado, Ohio) could very conceivably go to Kasich in a general election against Clinton provided the polls stay the same.