Let's face it, a significant part of the Republican base is white and xenophobic. Trump has established serious bona fides with those voters due to his anti-Mexico ranting. Those voters don't give two shits about McCain, they think he lost because he wasn't conservative enough and if his image takes a pounding, well so what? What todays "Republicans" adore is cocksureness, the ability to be wrong at the top of your lungs. Trump has it in spades. I just can't see the base deserting a True Believer for the sake of McCain.
The 21st century GOP has made rabble-rousing, rather than responsible adult policymaking, its core principle. This is the result.
Hey... You can't spend 30 years dry-humping the crazy and then act surprised when the crazy wants to fuck.