It's amazing that the vast majority of progressives always come around and vote for the lesser evil, yet we have to listen to this ranting every single time.
You are right that they should be pushing the Democratic establishment to do better, but there is a reason they don't. They know that progressives are willing to make compromises, but the establishment is not.
Biden would choose Trump over a progressive candidate. We know this because he did. When the Democrats moved the first primary to South Carolina from New Hampshire, they probably threw away the NH electoral votes, and they know they will never win SC. They would rather Trump win than face another progressive challenger.