There are two components to this question. Did many in the working class feel that Democrats had abandoned them? And is Trump's economic policy actually better for the working class than Harris's? I think the answers are "yes" and "probably no". However, voters don't listen to economists. If they're not happy with the status quo, they vote for disrupting the status quo even if experts tell them that that's a bad idea.
I suppose Sanders thinks that the working class would have supported a Democratic candidate who proposed a leftward (as opposed to Trump's rightward) disruption. My guess is that that isn't true and socialism is still a dirty word in America, but who knows?
Many senior Democrats were calling on Biden to resign long before he got covid, but he repeatedly made defiant announcements that he would never resign (and Harris supported him). He's the guy who said that he would only resign if God told him to! His covid infection appears to have been mild (lasting less than a week), and he resigned not because of it but because pretty much the entire Democratic establishment (led by Pelosi) told him that he must.