I think we're saying the same thing. He's spiteful BECAUSE he knows the DNC will defend him and because he isn't scared of losing his seat. He's mad at people being "annoying" by attempting to hold him accountable, and he is accountable to nobody. Hence he punches left to try to shut them up
But if he were to lose, establishment Dems who remain in positions of power wouldn't move to back a progressive replacement next election cycle. They would blame the left again, refusing to learn the lesson you want them to learn, repeat cycle into an infinite death spiral for democracy.
Lose the general*, I should clarify. I 100% support a progressive primary challenger against him. But I don't think letting a Republican win one of NY's Senate seats would move the Dems in a positive direction.