No one seems to think that the nominee in 2020 will be Donald Trump.
Anyway, I think Ted Cruz simply refuses to be a phony.
You can "not be a phony" and meet the minimum "I'm voting for the ticket." requirement of party loyalty. I could care less personally as to whether or not Cruz endorses Trump, because Trump's campaign is less endorsement-dependent than any campaign I have ever seen. But as a prognosticator (to the degree that I can call myself that), Cruz made a HUGE error. The "vote your conscience" mantra hurts Republicans other than Trump, and that is not lost on Republican regulars whose partisanship is not singular. Cruz went about this all wrong, and he's going to pay a huge price.
Being loyal to a party rather than your own beliefs is being a phony.