“The underlying assumption is Trump will drop out. Why not Cruz?”
Do people really think Trump would want to put up with all the BS he would have to as a President for 4 years?
Of course. He would revel in it. I think he's motivated by two things. First, he really does love this country and wants to see it "great again". Also he is near the end of his life, and he's looking at his legacy. He's done it all in terms of business, so what's left? There's nothing bigger than being president, and he wants to be the best one ever. It's in his nature.
Have you considered what Trump had to put up with to become as wealthy as he is?