I've got a few Trump supporters in the family, and they'll all leave the box blank and just vote down-ticket (if they go at all) if Trump isn't the nominee. They held their noses for Romney, but since he lost, it's a given that Kasich would lose too. Since he'll lose anyway, if he's at the top of the ticket, they're going to send a message to the party that they could've avoided this had they picked Trump.
Basically, if you're the party, you have to choose between 1) Trump, and you get blown out, but send a message to your irrational base that they're wrong about what the majority wants or 2) Not-Trump (doesn't matter who), and you kick this can down the road and deal with it again in 2020.
They'd be idiots to pick 2). They were idiots to do it with Romney. Should've sacrificed some Tea Party guy to Obama's second term (burn his incumbent advantage on a loser anyway) and then try to take the WH now against a non-incumbent.