A single progressive president means they get to name the DNC chair and a bunch of voting positions.
It's literally that easy to take over the party.
Obama just didn't do it because he didn't need the party after they turned on him for opposing Hillary.
If he'd have rebuilt it, we'd have a functional progressive party planning decades ahead already. And trump would still just be that guy from the Mac Miller song. The SC would be a progressive majority. The situation and Gaza wouldn't have turned into an open genocide, COVID would have been handled appropriately.
It's not some insurmountable task, but it gets harder and harder every cycle.
By all rights we should have had protests in the streets calling for Biden and the DNC leadership to step down for stealing NH's delagets. But not enough people had crossed their personal lines by then.
If we'd have had the fight then, we'd have had a full primary almost to figure shit out.