I do not think Cruz made the decision based on political expedience. I think the politically expedient thing for him to do was to have endorsed Trump. No one would have faulted him for it (especially because of the pledge). And Cruz’s endorsement or lack thereof is not affecting the outcome. If anything, his failure to endorse actually cast Trump in a sympathetic light and might benefit him.
Indeed, all the reports seem to be that trump new he would not endorse him, and he let him speak anyway. In short, of course he wants to run in 2020, but not endorsing trump will not help in that endeavor.
Anyway, we are certainly living in interesting political times for the US. No matter who gets elected, 40% of the country will despise the president. Its a little crazy.