That's the one scenario that has gone undiscussed.
We've got the most likely scenario, which is Trump runs for reelection and, if the economy holds up and he hasn't destroyed the world, he wins. That would suck. But, these things are cyclical and its difficult to unseat an incumbent President. Besides, Trump accurately reflects the zeitgeist of the people out there in the sticks and Republicans may say they won't vote for him but they will.
Then, you've got Trump runs for reelection and loses, another possibility albeit less likely. That is probably some combination of Russia scandal, a bad economy, a war gone all wrong, a gigantic natural disaster recovery effort botched, and a really extraordinary Democratic challenger. Could happen.
Then you've got impeachment and removal from office, or resignation. I don't see that happening but if it did that would be an extraordinary development indeed. I don't know what would play out during such a destabilizing event such as that. I do know it means President Pence and not necessarily imminent victory for Democrats. That's the reason I hope he stays in there.
Then there's health. He's an old man with bad habits. He could get terminally ill or drop dead.
But what if Trump just... checks out? Just says.. fuck it. This wasn't any fun, he's going home. AFTER he flies around the world in Air Force One promoting golf courses or whatever. That scenario would allow him an exit without defeat. Being an ex president is better than being president. More importantly, it then throws the political world into overdrive, as now we once again have an open seat. Probably with the most candidates we've ever seen on both sides.