What makes you think they've already won 2020? Trump only won in 2016 because he was running against the very definition of an establishment democrat, and one who was seen as corrupt and unpalatable by many voters. Even then, he only won because of the foibles of the electoral college system. Given the promising turnout from the midterms, a Trump victory (while certainly possible, mind) seems far from a foregone conclusion.
I guess KB's comment below demonstrates why the Republicans have already won 2020: The Democrats still can't figure out who they are, what they want to do, and 8 of the 10 people whose names you recognize running for the Democratic nomination are completely featureless, feckless, and unable to pull together any sort of compelling message. Meanwhile, the Republicans are voting for Republicans. It doesn't matter what the middle aged white guy says; as long as there is an R next to his name. That's the battle. And the Dems have lost it. Again.