I see it like this - Trump is most likely gonna win the nomination but obviously the establishment hates him. And Hillary will probably win the democratic nomination. So they back him, he wins, and the worst case scenario happens. The republicans hold the office for 4 years but soon as Trump is out liberals will come out in droves to make sure the GOP comes nowhere near the White House. It'll be eight years removed from a "real" Republican being in office and if Trump is elected president, I can't imagine they'll be happy with the results no matter what. It would take a LONG time for their base to regroup and rebuild if Trump is elected and worst comes to worst.
Whereas you can sandbag Trump if he wins the nomination, he'll be pissed, probably run as an independent, and the Republican base will split and they'll probably lose the election. But come 2020, the GOP can refocus and actually not trot out an embarrassment of candidates for the primaries.