It is becoming increasingly clear that Ben Carson, while a fine person and a perfect fit for several cabinet posts, is just not ready for prime time yet. The same is true with Rubio except that he is a little weasel and may not even be a good fit for any cabinet post.
It just makes sense to have four years of President Trump, after which Cruz would be the most likely person to follow. Why? Trump's the only one with the bluster to turn things around quickly enough to have a chance at saving the US.
I envision a serious, SERIOUS push by the GOPe to make Rubio their guy... it's happening right now.