You just misread the reasons why Trump was elected, and give too much credit to his outsider schtick. There is no Democrat Trump waiting in the wings to take down Biden in 2020, you'll see. Donald Trump's do not grow on trees.
Biden may be weak, but the Democrat competition is even weaker, and that's what will carry him to the nomination if he doesn't commit political suicide. Obvious frontrunner is obvious, some of y'all are just in denial and think an outsider schtick is an automatic winner.