Except I'm not using the word "impossible" anywhere. The first Trump election was extremely improbable, not impossible (Nate Silver had him at 29% on the day of the election). He's become significantly less popular since then. It's important to understand how 2016 happened. He had a path to victory in 2016 that required him to win some recently economically disadvantaged swing states and combination of willingness to speak about economic issues (poorly, and he lied a whole lot in his trademark fashion, but he spoke about them while Clinton kind of wasn't ****ing willing to in anything other than weasel words) with a side order of racist dog whistling and he swept the bag of them. Since then he's lost a pile of support in those same states because they realize that he lied his face off about giving a **** about middle america's economic issues and there has been a surge on the democrat side of things in legitimate candidates who are willing to talk about income inequality and related issues.
He has the same path to victory in 2016 except he's going to be carrying 4 years of lies and political baggage when he goes to the same people and does what? Admits that the economy passed them by, but give him another shot? Nope. He's going to stand in front of them, and proclaim that America was made great again by him and him alone. He's not some unknown outsider to these people anymore who can sell a vision of how incredible a leader he is. He's a known commodity and the heavy majority in most of those swing states finds him wanting.
Can the Democrats bottle this? Absolutely. Failure is in their ****ing DNA. But I think it's worth pointing out how hard a big chunk of the party has shifted to focusing on income inequality and related issues. 4 years ago it was Bernie with an unknown Warren as the only real champions of these issues. Now it's the majority of candidates who are basically repackaging Bernie's 2016 talking points for their own use...and if that fails, white people love Biden. Trump's kryptonite is having his white base given a charming, funny old white guy to measure him against.