Why endorse at all? Why not just do what he promised, just keep his mouth shut and focus on his plan for running the city?

Obama 2.0, that's why. Dude is already planning for the future.

We'll see if he is able to convert into something for anyone other than himself, but right now, by centering everything around purely himself without anything to show for it, without even honoring a vote, he is setting up the political machine to be around himself and no one else, which is mighty dangerous for someone who is supposedly challenging the system. Even Lenin at least entertained a pretense towards democratic decisions within the party, even if he liked to stack the deck. At this point DSA is literally more 'authoritarian' than the Bolsheviks.