Not true. The midwestern states that traditionally voted Democratic are snapping back that way. Trump barely won those midwestern states in the first place. Look at the races in MI, for example; Democratic candidates are crushing in MI-GOV and in house races, too. Baldwin is up comfortably in WI.
Trump had to have everything fall juuuuust right to win, and remember, he was voted against by three million people in raw numbers. He won three midwestern states by fewer than what, 20K votes? If the Green Party didn't exist and those voters voted D, he would have lost those states.
I won't say Trump will definitely lose in 2020, but if fundamentals don't change, he's going to definitely lose and, depending on the charisma of the candidate, he will likely get crushed.
It's also worth noting that in FL, which Trump won, the white trash is trying to do their rural surge shyt again, but this time Gillum and Nelson are still winning big. If Gillum wins that FL-GOV race by six-to-eight points, consider that a harbinger for 2020 - Trump will not win FL again, either.
Finally, the Democratic bench for 2020 is fine. Gillum will probably be too busy governing FL, but O'Rourke will likely a) lose and b) come close enough that people look at him as nationally viable, and people seem to be excited for a couple of our potential candidates based on what has been written about their crowds (like Kamala Harris, which surprises me).