If Bernie actually had the momentum and numbers to start his own party he wouldn’t have to worry about the DNC shenanigans.
This is a good point and it's exactly why the establishment/moderates don't like him. He wants to use the Dem platform as a means to an end but wants to push the party further left and in a different direction from where they are now. From Bernie's perspective, it makes sense. He knows he'd never win as an Independent and would only split the Dem vote and give Trump an easy win. The flip side is that he's getting all the shine reppin' the Donkey but doesn't really give anything in return.
On top of that, does he really have strong coattails. Winning seats in NYC, Cali etc., isn't going to be enough. He's going to have to win in flyover country because if the Senate doesn't flip.......nothing he's proposing will get done. I'm not sure why people don't understand this. All the ideas in the world don't mean shyt if the GOP just decides to stonewall you. He's gotta win and then have candidates that can win everywhere down-ballot.