Cruz could actually get a Trump endorsement via the Jeff Sessions connection. I can't imagine Trump/Sessions would endorse the traitor Rubio.
Cruz would motivate the base, the mainstream/establishment voters would even support him (because at least he's not Trump), AND he's the one guy who could conceivably get a heartfelt Trump/Sessions endorsement.
Basically, Cruz could keep Romney's coalition, but then ADD ON a motivated base and new white working class voters via Trump. He would beat Hillary like a drum with that coalition.
He's always been the right compromise candidate for many months now.
If the GOP instead performs the convention steal to nominate Romney, Ryan, or Rubio, it's over for them as a Party.