My random thoughts. If Trump and the GOP don't deliver those in the Senate will go down with the ship and it may impact Cruz negatively in the long run in 2020. I don't forsee Trump running again, but certainly Pence will. I'm not so sure that Cruz would run against Pence, a sitting VP; he may join the ticket at best. 2024 would be Cruz's year to run, but that's a very long ways off. Obviously he was under enormous pressure to state that he was going to vote for Trump and campaign with Pence. Trump knows that Cruz is going to be a thorn in his side in the Senate should he not adhere to his promises. Cruz isn't going to be offered a cabinet position, so the only way that Trump can get him out of the way right now is to offer him a seat on the bench. If Cruz doesn't accept, Trump may keep his promise and see to it that he won't win re-election in the Senate.
Cruz may very well look at being a Supreme Court justice as an opportunity that comes around once in a lifetime at best. Because of the ages of the other justices there will be another opportunity probably sooner than later, but there is not guarantee that Trump/Pence will still be in office, nor is there a guarantee that Trump will appoint another conservative.