Another thing electoral movements can do is improve conditions enough to where change that previously wasn’t possible becomes possible. Think of a mild reform like forcing police to use body-worn cameras. It certainly didn’t solve all problems with policing in capitalist societies, but it did open up a lot of avenues for changing cop behavior and agitating for further improvements.
More broadly, I think we have to be somewhat agnostic about what electoral movements can and cannot do. No one has ever brought socialism to a bourgeois democracy, or to an economy as developed as that of the modern imperial core. The closest examples to accomplishing that come from Latin America – Chile under Allende, the pink tide early in the 21st century – and were done through elections. There are no historical successes in anything close to the conditions of, say, the modern U.S. No one really knows what will or won’t work.
The 2020 primary was more complicated than that.
Biden was dead in the water until the eve of Super Tuesday. Bernie had a strong lead coming out of the early states despite the best efforts of the media. Then you had Obama coordinate the centrist candidates to drop out and endorse Biden, and the media was happy to treat that as normal and good. And all that might not have been enough, but then Covid hit, Bernie suspended his campaign, and Biden told all his elderly supporters to go out and vote anyway.
The media is a big factor, but it was not calling the shots in 2020. That was Obama more than any one person, and all the weirdness of the early pandemic was a huge X factor.