Name a challenger who got national traction in a primary who was a complete no-name like Austin Peterson. You can't because they don't exist.
But we've never had a president elected before who wasn't endorsed by more than 20% of his own party's Senators. 
The modern primary system has only existed since the 1970s, and during that time, up until Trump, we never had someone elected despite having this kind of resistance from a non-trivial minority of his own party's elite. When said president now runs for reelection, the dynamics will not necessarily be what we're used to.
So I'd say we're in uncharted territory. Maybe they'll all be cool with nominating Trump again in 2020, I don't know. But if there are still some disgruntled folks at that point, I don't think it's obvious that they'd be able to recruit some bigger name candidate. You're asking someone to turn their lives upside down and put a target on their back for a presumably doomed campaign. Amash might do it, for example, or maybe a former governor like Kasich, but otherwise, you might just have someone like a McMullin or a Peterson.
And keep in mind, the folks looking for someone like McMullin last time around tended to be more media types (like Erick Erickson and Bill Kristol), moreso than politicians. And even you seemed to be suggesting here:
https://uselectionatlas.org/FORUM/index.php?topic=262379.msg5607740#msg5607740
that some of the politicians who didn’t endorse Trump last time won’t endorse him again. If they’re not going to endorse him in the GE, then they of course also wouldn’t endorse him in the primary, leading to more discussion than is typical about what alternatives exist.
you seem to drastically be overestimating the influence of Erick Erickson, Bill Kristol, Jeb Bush and John Kasich.
Austin Peterson is a no one, if hes the only challenger he'll be treated as such.
Poll
