Also important to note is that while Trump himself might not run, especially considering his age, he might start a news network like he planned in the event of a 2016 loss. A candidate from the populist wing with his blessing would effectively be a Trump surrogate as a Vice President Harris run would be for Biden. Basically, a rematch without actually being a rematch.
The important thing for the MAGA movement is to keep the momentum going. Trump needs to be front and center, and to keep rallies packed and MC for the appointed successor. The "elitists stole the election from us and we need to get our country back" message will resonate with a lot of these people.
Of course, the MAGA base could just get fatigued or stop believing in Trump because he lost, and they like Presidents who don't lose.