There is no proof that either Cruz or Rubio would do better than Trump in the general election.
Cruz is an ideological candidate of the GOP conservative wing. If we look at the past results, we can only expect him to do extraordinarily bad in the general and to impact the downballot races somewhat. No guarantee the GOP will keep the Senate with Cruz, though the House should be safe.
Rubio is a lightweight, but he would probably be better than Cruz, again judging by past election results. The GOP might keep the Senate with him.
Trump is a wildcard. he loses a lot of votes in solid Republican states and in solid Democratic states. Some of the traditional swing states are out of reach for him. But he plays very well in a number of key swing states and in some traditionally Democratic states. He may expand the map for the GOP.
The thing is, with the map being the way it is, I can't imagine a GOP win with any of the candidates save Trump, for he alone can expand the map to states which normally would be considered safe or likely D (PA and MI).
It will be interesting to see if what you are proposing becomes reality.
Trump is like someone driving with one foot flat on the accelerator and one foot on the brake doing 100mph.
You just hope he does not his a tree, but he certainly feels a little off balance at times.
