Maybe Kasich is, but unlike Cruz and against everything Trump stands for (ME! ME! ME!), both he and Rubio have records of working across party lines to achieve policy aims. This includes making concessions in order to achieve some perceived greater good... you know, the way politics is supposed to work.
I think you're underestimating Trump. Most R primaries/caucuses have had record turnout, and Trump is winning the "first time participant" vote. So I don't think he's motivating a "small sliver". I think he's motivating a larger base of generally disinterested voters who believe that their standard establishment candidate doesn't speak for them.
I think Trump is also turning off a lot of voters like me, but I believe I'm in some minority of people who would rather vote Hillary or stay home. Anecdoctally, in discussions with family members who are all either Rubio or Cruz supporters, every single other person will vote for Trump over Hillary or Sanders whereas I will hold my nose and vote Hillary. I think the former is far more the majority of conservatives and right-leaning independents than the latter. On the whole, if Trump wins the nomination, I think he'll gain more votes from "normal non-participants" than he loses from folks like me. But that's not enough to win; he'd need to both motivate low-info/low-participation voters (as he does) and win center-right votes. Ironically, he's probably the most center of any of the clown car, yet he is completely unelectable for me.
I agree with your overall point that unless Trump somehow motivates people like me to show up and vote for him along with some others, he won't win the GE against Hillary, thankfully.