Trump may be a gross individual, but I still don't see him losing 2020 unless the Dems decide to go for a moderate candidate who can actually pull some Trump supporters away and gain widespread independent support. A progressive candidate won't be able to cut it.
A moderate is what they offered in 2016 against a proclaimed "Drain the swamp" radical vision. Recall Bernie was the answer to that, but got pushed aside by the DNC? Trump is still radical to establishment politicians, as was a Bernie Sanders. America wanted a change, but they invited not only a wrecking ball to the system, but against better judgements someone who does not care about the law or the constitution, but is only absorbed in his pathologically narcissistic personality disorder, and has from the beginning poised himself to be a dictator, should the situation call for it, or he for the situation.
So, while in principle, yes some "moderates" sounds appealing for the reasons you say, those same "moderate" republicans did not choice someone on radical vs. moderate grounds. They choice, against everyone's own best interests, to vote party over sanity.