I think the best way for progressives to show that they can win elections on single-payer, $15 minimum wage, etc. is to rally around progressive candidates and win. If that was a message that would have worked in GA-6, then someone should have opposed Ossoff from the left in the primary. (I don’t know much about the other Democrats who ran in the primary and their policies, but I don’t think any got more than a few thousand votes.) This after-the-fact bemoaning that the Democratic Party rallied around the centrist candidate who got the nomination seems sort of pointless.
Run a progressive, and win. That’s the sort of argument that the DNC will listen to more than saying now that if you had done it our way, the result would have been different. We will never know if someone to the left of Ossoff would have done better. (Full disclosure, I almost always will vote for someone with a D next to their name in general elections, but my electoral politics activity at the moment consists of working for a progressive City Council candidate who is opposed by an old-school Brooklyn machine Democrat.)