If Democrats indeed want to win again, they have to go further left.

The basic problem is that the Democratic Party isn’t at all inclined to go left. The people who run it and fund it want a centre-right party to be the only alternative to the far-right party. Left-wing politics doesn’t funnel the money to where they want it to be. So the best they can do is tease people with a vague promise of progressivism, in order to win votes, but with no real intention or commitnent to deliver on it.