Democrats won't learn anything from this election, just like they didn't learn anything from 2016, because there's no need. People will get sick of Trump and vote for the Dems again. And they'll get their turn. Wash, rinse, repeat.
The duopoly is a power sharing arrangement between two corrupt, entrenched parties. Each party is happy to "lose" every term or two, as long as their retain their privileged position. There's no need for them to offer competent leadership. As long as they block out any real competition, they can keep up their charade up and running. The question is, how long will we keep falling for it?