How would your scenario above be any different than the 2016 election? I see virtually no one who voted for Trump, even grudgingly, abandoning him in favor of a fictitious GOP opponent or any democrat candidate.
His scenario is extremely flawed because he's leaving out several key elements, much the same as the MSM.
The extreme negativity that the MSM is pushing is not actually hurting Trump, it's actually hurting the MSM as it finally shows their true agenda. And as the average American, those left in the fly over country see this it only emboldens them more to support Trump.
Same with the Alt Left Radical Progressives, as they go further and further off the deep end in feeble attempts to smear Trump they are only exposing themselves.
And the establishment republicans? After watching their self centered lame attempts to govern and run congress, why would the majority of conservative voters want to see the republican party attempt to nominate yet another establishment republican? The election of 2016 should tell anyone all they need to know about how people feel about establishment politics. And the result of 2020 is not going to be "Oh crap!, let's get Jeb to run!" or "Geez, maybe we can get Mitt?"
If the republican party tries to get an establishment type nominated to push out Trump it will blow up in their face.
Lots can happen in the next 12 months and many dynamics can change, but I seriously doubt that Americans are going to start embracing the establishment again.