Barring a brokered convention during which the party decides to say “fuck the primary voters,” Trump will be the nominee. But I don't see how he wins the general election without something seriously weird happening. Trump has Carter-level unfavorability numbers, and those are likely to go up after the vetting that comes with a nomination. Clinton, meanwhile, has already alienated everyone she can. She's a known quantity who most people made up their mind about a long time ago. Her unfavorability numbers can only come down (but they won’t—she’ll make friends with some undecideds during the campaign, but she’s not going to be making any conversions).
Plus she has an advantage that numbers don’t reveal: voters like the Clintons way more than they are willing to admit. Every time one of them is on the ballot, the opinion polls and exit results always show that more people are willing to vote for them than are willing to admit they voted for them. So if we’ve got lifelong Republicans coming out and publicly declaring their willingness to vote for Clinton, think of what that says about the silent majority. We’re going to have a lot of people holding their noses at the ballot this year.
Then there’s the possibility that Sanders ends up the Democratic candidate. His favorability numbers would come down a bit in a general election, too, and his unfavorables would go up. If Clinton is right about one thing, it’s that his current numbers aren’t a great reflection of what the voters will think in November if he's the candidate. The Democratic debates have been buried, with only the occasional highlight reaching the average voter. So while I think Sanders could win over a lot of undecideds, he’s going to alienate a lot of them at the same time. He has some overlapping support with Trump from voters who just want change, however, so that could actually help him out quite a lot if he makes it to the general election.
And that’s not even considering the possibility of a third party run from a Republican other than Trump. If the GOP leaders decide that losing the White House (while potentially holding onto Congress) is better than letting Trump become president while holding their banner, then they might decide to split the electorate on their own. It would be a bold move—and one that could be easily interpreted as noble, thereby winning over moderate support for current down-ticket candidates and for future Republican presidential candidates. I can't see that happening without a brokered convention, though. If Trump gets enough delegates in the primary to win outright, the party will be too scared to challenge him that way.