Neither Cruz nor Rubio is going to run against Trump again except in the unlikely event that (post-Dems taking over Congress) Trump tacks so far left that conservative media like Fox News turns against him. They're both waiting until 2024 to run for president again.
OTOH, I can actually imagine Rand Paul deciding to challenge Trump, despite all the sucking up to Trump that he's currently doing. It's not likely, but it seems more likely than either Cruz or Rubio running. Cruz and Rubio actually have ambitions to lead the party post-Trump, whereas Paul is never going to be able to do that, so he has less to lose.
I think you make a very good point, but if it seems clear that Trump would lose renomination I can't imagine either Cruz or Rubio choosing to remain on the sidelines.
I guess that's what I'm saying about "if Fox News turns against him". Trump is not going to lose renomination unless something happens that prompts the conservative media complex to turn against him. In that scenario, sure, Cruz and Rubio would run. But it seems very unlikely to happen, IMHO, so I think their chances of running remain slim.
