Short of him being removed from office by impeachment or indictment (if the latter is even possible), I do think Trump will be the 2020 Republican nominee.
Even now, knowing all that we know about him — the criminal activity, the corruption, the lecherous creepy-old-man behavior, the 4-hour workdays, and just the general incompetence and ignorance — despite all that, his approval ratings among Republican voters are still in the 85–90% range.
This is why you haven’t seen the Republican in congress turn on him yet. They want to, desperately. I believe that they would like nothing better than to oust Trump and have Pence finish out the term.
But their voters won’t have that.
Republicans have been party before country for a long time. Now they’ve gone even beyond that — now they’re Trump before party.