We have to think of these things first up here. There are some people, even some Republicans, who think someone should challenge Trump in the primaries. There is a claim that it's never good for politicians to think such things are their automatically - okay, I see that; and there is also the claim that if no one discusses the major issues from a Republican perspective, then the Democrats will control the narrative. I see that as well, especially with a media who would love to spend months saying "Last night, candidate Hillary Clinton/Beto O'Rourke/Joe Biden/Elizabeth Warren hammered the Trump administration hard on the issue of______"
Still, I suspect most people making that suggestion mostly want to sow discord. Yet there is another point I think could be added: What if Trump decides halfway through he doesn't want to run again? What if he has a stroke, or dies? Do the votes revert to the second-place finisher? Or does that just guarantee a second-ballot convention?
I think Mike Pence should run against him. He could offer to make Trump his Vice President if he wins.