I've said this before, but let's look at it.
Hillary and Trump overlap a little in the middle, with moderates, but most of those will stay home or be split, so we can ignore them.
So, Hillary has the Democratic base and women. She loses some Bernie-type independents and left-wingers, along with some men.
Trump has the Republican base (conservatives, evangelicals), independents, men, and many Democrats. He loses Establishment Republicans, and women.
The Dems he pulls cancels out the RINOs he loses, and that could get even better if more Establishment types support him once he is the nominee. So, he has:
Hillary's moderate RINO pull is canceled out by Bernie-type Dems seeing her as too dishonest and untrustworthy. So, she has:
With that in mind, let's look at the electoral college. Hillary can get Democratic bases (West Coast, NE).
Trump can get Republican bases (South, Texas, Midwest).
He may be able to get a couple NE states, she may be able to get places like Utah.
Now, we come to "swing" states like Virginia, Ohio, Pennsylvania, and Florida. Trump can get the disenfranchised coal miners and whites in Pennsylvania. He can get the independents, Dems and base in Virginia, Florida, and Ohio. It's easy to see him winning 3/4 of the swing states.
Now, say we have Cruz. He is conservative, so he gets the base, but hasn't a chance of getting independents and Dems. Loses in every swing state. He loses to Hillary.
Kasich? Gets moderates and some independents, but if the nomination is stolen by a RINO, the base will revolt and stay home, or write him in.
Ryan? Same as Kasich, but even worse.
Romney? See above.
Trump is the only Republican who can win.