If the GOP doesn't want Jeb! (peaked at 17%, the same as Scott Walker and Rand Paul...Chris Christie peaked at 20%) now, why would they want him in 2020?
I mean, Mitt Romney was roughly as or more competitive in 2008 nationally. And led in both Iowa and NH until about December. And he still got shoved around for a while in 2012 by Rick Santorum and Newt Gingrich of all people. (As late as mid-March the national polls were Romney 35% - Santorum 29% - Gingrich 14% - Paul 11%; on Feb 28th Santorum led Romney 33-31.)
The failure of Romney, Rudy and (compared to the current field) Fred Thompson (and ultimately Johm McCain) didn't lead to more moderate candidates in 2012. Nor did the failure of Jon Huntsman, Tim Pawlenty and ultimately Romney (arguably all to the right of Rudy and pre-epic pandering mode Johm McCain) in 2012 lead to a more moderate field in 2016.
And the "establishment" is getting more fractured and more gunned down or dying off. Taking the RCP average you could argue that the "Tea Party wing" has two-thirds (combined polling of 68%) of the candidates (Trump, Carson, Cruz, Fiorina, Huckabee, Paul, Jindal, Santorum), and both Kasich and Rubio (combined 12%) rode the 2010 wave into office. The "establishment/moderates" has at best 18% in the polls right now (Rubio-Jeb?-Christie-Graham-Pataki) and moving Carly, Kasich and/or Santorum into that group doesn't get them much past 25%.
One could argue the "establishment" peaked in either September 2013 (Christie-Rubio-Jeb! at 42% vs. Paul-Cruz-Walker-Santorum-Jindal at 33%) or September 2014 (Christie-Jeb!-Rubio at 31% vs. Huckabee-Paul-Cruz-Walker-Santorum-Jindal at 40% with Kasich at 2%. If you slid Huckabee and Kasich over to the "establishment/moderate" side it's 43% vs 29%. Santorum is at 3%.)
I'd expect an even less moderate field, especially if Hillary wins and is polling strong in 2019. Candidates aren't going to want to enter a likely losing race. Outside the bin Laden bump, Obama had a negative job approval from basically July 2010 to January 2012. And the Republicans still didn't churn out a field of big time candidates. Michelle Bachmann, Herman Cain, Gary Johnson, Rick Santorum, Thaddeus McCotter, Buddy Romer, and Jon Huntsman was the field with Romney and Gingrich before Perry finally entered in August 2011. Haley Barbour almost got in, and Trump played hard to get. But Christie, Jeb!, Daniels, Huckabee, Palin, Petraeus, Scott Brown, etc. all passed and pretty quickly. Even a guy like Pataki did.