I think people are underestimating how truly disastrous a Trump candidacy would be - Trump being the candidate pretty much assures they lose both the Senate and the White House vs. maybe a fighting chance to hold onto the Senate if Trump goes 3rd party.
While this might be true, remember that if it comes to a Trump 3rd party bid he's not going to be running to win. He's going to be running to burn the GOP to the ground and to do as much damage as humanly possible.