Trump falls just short of the delegates needed to win on the first ballot. On a subsequent ballot, the republican party nominates Rubio (or Bush, or Romney, or Ryan, etc). Trump, having pledged not to run as a 3rd party candidate, does so anyway. The republican party nominated somebody else despite the fact that he, Trump, got far more votes than anyone else, so why would he abide by a pledge he made to them? Probably, at this point, Hillary would win the general election.
If the republicans nominate Rubio, (or maybe one of the others), and Trump doesn’t run, the republican nominee would have a decent shot at beating Hillary. Maybe… What about this:
If Trump ends up with a solid plurality of delegates, but not quite enough to win on the first ballot, and the republicans nominate somebody else, and Trump doesn’t run as a 3rd party candidate, what are all those Trump voters going to do? They’re already angry. They already feel like they don’t have a voice (research is showing that feeling like you don’t have a voice in the direction of this country is the most accurate predictor of whether you’re a Trump fan or not). You take millions of angry people who don’t feel like they have a voice, and then you TELL them that they don’t have a voice by nominating someone other than the candidate that got the most votes. Someone other than their candidate, the one that was speaking for them, giving them a voice. Who got more votes than anyone else, by a large margin. You tell them: Your vote doesn’t matter. You don’t matter. Fuck off. We, the republican elite, will decide who your president will be. And you, you go ahead and go back to not mattering. You go back to being nothing.
I’m not a guy who sees revolt around every corner. And, even in the above scenario, I don’t think much would come of it, other than futile, impotent, raging, short-lived anger. But if ever there were a recipe for revolt, short of hunger and imprisonment, it seems to me that would be it.