Some of my thoughts:
* Bernie Sanders just doesn't have the luster he did in 2016, when he first became popular. Same as how Ron Paul didn't have the same luster in 2012 as in 2008. In the unlikely event that he gets the nomination, he will move to the center on a lot of issues.
* Biden's mental faculties are quite clear at this point. He is rarely able to communicate unless he has rehearsed his lines and sometimes not even then. Trump would easily be able to bully him in the Trump v. Biden debates like he did to Hillary; this is not true of Warren or Sanders.
* Elizabeth Warren's problem is that she is boring and boring-looking. She talks and looks like your leftwing aunt who lives in Portland or Seattle. She won't have any catchy slogan (e.g. "Feel the Bern!" or "Keep America Great!") and any emotion she tries to show will seem contrived. I actually think that Elizabeth Warren is the candidate who most resembles Hillary Clinton.
* RGB may kick the curb in the next year. This would of course be the issue that dominates the election and may bring a few million anti-Trump Christian voters out of the woodwork to vote for Trump. I think that number of voters is much greater than the number of untapped pro-abortion voters.
* The idea that hardcore lefties who don't like Joe Biden will still vote for him as a vote against Trump may not be true. The Libertarian Party had a good year in 2012 because it sucked a lot of votes from hardcore righties who didn't like Obama or Romney. A leftwing 3rd party could suck votes from Joe Biden.
* I definitely don't believe in the myth of the "Hillary voter who stayed home," the idea that many would-be Hillary voters didn't vote because they assumed she would win anyways.
* Anti-gun stances (Could that be the issue of 2020? It seems to be the most consistently talked about issue on the news, in the debates, etc.) will hurt the Democrats badly with Middle America swing voters.
* The U.S. could have some string of good luck in the next year (e.g. cancer cured or some other new invention, an AI revolution, a new bubble in some industry, Dow rockets to 40k, etc.) and it would boost the current president's popularity even though he doesn't deserve credit for it. Absent of this, I don't see Trump's approval rating getting as high as 55%+ and the theoretical ceiling is probably something like 65% anyhow.