He might run, but I'm just a teensy bit skeptical that the base will run into the arms of a tempermentally moderate Reaganesque conservative and softie on immigration after nominating a bomb-throwing, street-fighting nationalist just four years prior.
I see what you're saying, but I also think Trump primary voters are unique in the sense that many of them aren't very involved in the political process. So long as Trump doesn't run again after losing, I think it's possible many of his voters don't show up to vote for other candidates. Also, now that the establishment knows that someone like Trump can actually be nominated, I think they will work earlier and harder to unite behind one candidate, and Paul Ryan will likely be that candidate. I don't think you will see a bunch of establishment types (like this year with Bush, Rubio, Kasich, Christie etc), but rather many will unite behind Paul Ryan. It's not a guarantee Ryan is nominated in 2020, but I at least think he starts out as the favorite.