You mean drop out of the race before November? I've seen the speculation but I think it's wishful thinking. By staying in, he retains a chance to turn things around and pull off another upset and, even if he loses, he can spend four years complaining about election fraud and pumping donors for money by promising another run in 2024.
The bigger question that you might be intending is, what happens to the GOP after 2020 if he loses/drops out? The moderates are already leaving (some new super-PAC of Bush II supporters just announced that it would oppose Don) and demographics are working badly against the party as long as its reactionary base remains prevalent. That base isn't gonna suddenly wake up and decide that they're wrong, no matter how badly their candidate might lose in November.
Maybe we shouldn't even be talking about this stuff, interesting as it is. I don't want to jinx us.