The Republicans are getting more frantic about the prospect of Trump winning their nomination. Various semi-secret "meetings" are being held to consider increasingly desperate-sounding options – a Cruz-Kasich "unity ticket", convention shenanigans, rule changes, and so on – and that hardly can be interpreted as a positive omen. When a group of people priding itself on refusing to change finally gets around to admitting that there is a problem, it's likely already too late. But what if they do succeed in coming up with a means of denying Trump the nomination? Aside from those riots he has promised, the assumption is that Trump would continue as an Independent candidate. If he's as rich as he claims to be, certainly he could afford to do so (especially given that he doesn't have to pay for media exposure).
The problem with the Trump-as-Independent (or some GOP savior running as Independent after Trump secures the nomination) is the fundamental issue of ballot access. Every state and territory has its own rules and deadlines for who can appear on the November election ballot. The Republican Convention is scheduled for July 18-21, by which point 11 states' deadlines will have passed.
Go ahead and try to conceive of a scenario in which a Republican or Independent Trump wins the election without being on the ballot in Texas, Florida, and North Carolina. I'll wait.
That would leave two remaining options. The first is to run what would be at least partially a write-in campaign. Texas, with its early May deadline, at the very least would end up requiring write-ins. The second would be to use an existing third/minor party that has already secured ballot access. Even this Hail Mary strategy is problematic, though. The Libertarian Party leads current U.S. third parties with ballot access in 34 states. That's less than 2/3 of the states. The Green Party isn't going to help (and is in only about 20 states). The wingnut Constitution Party would be a logical home, but they're barely in a dozen states (they claim 15, although other sources list fewer).
The upshot is that Trump as an Independent, assuming that his quest for the GOP nomination is thwarted (if at all) at the convention in mid-July, would be running solely as a spoiler. He wouldn't even be on every state's ballot, and the idea that he's going to win an Electoral College majority with a mishmash of write-in and Independent votes borders on silly. In any scenario in which Trump and another Republican are both running it's inevitable that they will split the right-wing vote and lead to a Democratic blowout in the Electoral College. If you have wondered what the Republican Party's strength would be without the Tea Party, this will provide the answer.
I can't wait.