There isn't leftward progress, just a steady liberal trend. Biden is great if you're a liberal, he's been doing exactly what he said he would, even if he hasn't gotten everything he wants done. But for an actual leftist? Jack shit to move America to Socialism.
The leftist reason to vote for Biden is because he isn't a fascist, that's it. Biden is still just a Capitalist and won't move America anywhere to the left. In case you haven't noticed, the far left have been holding their noses and voting liberal for as long as there's been leftists.
The actual way to move America to the left is pressure from grassroots movements like Unionization.