The Republican Establishment will insist that Trump pick an Establishment person as his running mate in return for its support. It will give Trump the same ultimatum that it gave Reagan: Ford or Bush. In this case the ultimatum will be Bush, Rubio or Kasich.
Just a hunch, but I dont think Trump bows down to ultimatums
I am struggling as to how to believe that the Establishment is going to be relevant in that decision.