Bernie, who is conveniently not a Democrat when people (centrists and leftists) want him to not be a Democrat, wins Democratic primaries. He’s not even in the party and he’s the supposed example of true leftism opposed to neoliberal centrism, and he still wins the primaries. He could go it alone in a three way race like you wanted, and the all-powerful DNC could rig his primary so he’d be out of their hair for good, but weirdly both don’t. Because in the end it’s just the votes that matter and winning a primary to clear a lane and then winning a 1-on-1 against the conservatives is easier and more effective.

And no, those things aren’t rigging. Opposing coalitions of centrists using their social influence or even antagonistic organizations spending money on advertising does not mean the vote is unfair. And neither of those things go away if you just decide to make a new party. However you label yourselves, you need to be ready and able to beat that.

And people do! AIPAC wanted Omar out too. The party establishment never wanted AOC to win. They both had less money and less influence than the forces arrayed against them, and they won by just doing good politics and convincing regular people to vote for them. Because whatever money and machine politics was against them, the thing that counts is still just votes.