Not so clear that it wouldn't involve a fight. Remember 2000? I sure do.
As I said, I'm sure he won't want to go, but that's diffferent from installing himself as dictator-for-life. I remember 2000 - as I recall Bill Clinton did not try to retain the Presidency, and while there was a legally contested fight that wound up in front of SCOTUS over a fraction of votes there was not an armed insurrection, and despite some bumps there was a peaceful transition of power.
One way or the other I don't think the race will be anywhere near that close.
He is riling up his base for a reason. I firmly believe they will rise up in armed insurrection if he loses in 2020, or in the unlikely event that he is removed from office in some other way.
You have less faith in the strength of our democracy than I do. Should he lose (and that's not a given) I'm certain many in his base will refute it in some way, but an armed uprising beyond a few dozen extremists seems unlikely to me. There are many who believe Gore had the election stolen from him, yet even then the transfer was peaceful. As I said before, should DJT lose in 2020 the most likely scenario is that he lost both the popular vote and in the electoral college. Ostensibly that would put the support of the majority and the clear constitutionality behind someone else.
Dictators need the support of the military to stay in power against the will of the majority of the people. I don't see that scenario either - most of the upper brass seems sick of him (remember how his relationship with McMaster, Kelly, Mattis, Flynn, McRaven, McCrystal and others have turned). They follow his commands because it's their sworn duty, and they take the CoC very seriously. In the wake of a loss that will be gone.