Nope. They would have just stayed home like they did in 2012.
Trump has won about 40% of the primary vote so far. That's about 3.2 million people.
Do you mean to tell me that all, or a large share of those 3.2 million people would have stayed home if Trump wasn't on the ballot?
I've seen numerous Trump supporters say that Cruz was their second choice. So it's a good bet that they would have voted for Cruz.
I've not seen one Trump supporter say that they would have voted for Bush.
So I think a reasonable conclusion is that if Trump wasn't in the race, then Cruz would have been easily winning and that Bush would have been where he is now, which is nowhere!