That establishment vs. conservative thing is slanted. A lot of the time, region has more to do with it. People on the coasts who aren't members of any Establishment vote differently from people in the South (who may not be that conservative on some issues).
In this case, though, there is some sense to it. It's hard to see Rubio having much appeal outside "Establishment" circles. Like Jeb, he doesn't light many fires with the public. He'd be a sort of "anti-somebody else" candidate -- and those guys never fare well.
But really, for Ted Cruz to come out at number one, Trump would have to fail really spectacularly -- and the noise when he falls could make major trouble for the party.
.
Trump has already failed with the majority.
Had he not failed, they would already be in his corner, considering all the free face time and publicity the MSM give him.
.