Of course nothing is a fait accompli. If the Ds are that stupid in 2020, after 2016, they deserve to be shot into space. They need to work super hard on turn out and effective messaging.
But...
1) US politics is largely driven by negative partisanship
2) Dotard was a minority president, with historically high and persistent negatives.
3) In 2018, despite a reasonably good turnout in midterms by the Trumpanzees, the GOP was smushed in the House
4) Bitcofer: 2018 was driven a surge of D turnout driven by negative partisanship - the revulsion is very strong
5) Apart from Alabama, the Senate is pretty much all defence for the GOP in 2020
5) 2nd term election is usually a referendum on the sitting president.
6) For both camps, Dorito has basically met or exceeded expectations (very positive and very negative), which does nothing but exacerbate negative partisanship
7) Trump is underwater in a lot of batttle ground states (not talking about national polls here...) - if he continues to motivate negative partisanship turnout, then he is cooked. It is just math.