I don't see a serious Republican challenge to Trump in 2020. Anyone who could beat him will keep their powder dry for 2024.
I agree, which is why it is important that Trump decide for himself to not run again. If he does, it will attract an independent candidacy that will doom his chances at re-election, such as they are.
I'm not alone in believing Trump will decide not to run. He's a non-politician, with extensive interests in businesses that he needs to gets back to in order to make sure they can successfully transition to the next generation. He will be one of the oldest Presidents in history. And (at least as things now stand) he can claim real victories and real credit in changing the priorities of the GOP. I believe he will plausibly claim that his work is done.
The biggest threat to this (IMO) rosy scenario is the Democrats, who've run against Trump to the exclusion of little else for two years now. The last thing they want is for Trump to voluntarily step down in favor of new blood. They are counting on Trump as the nominee, and they know that to push his buttons to goad him to fight yields the very best outcome for them.