If some shitheads get rich, who cares? I certainly don’t want to help them get rich. If I get rich doing that, I’ve failed.
And of course this is all to say nothing of the extreme selection bias inherent to your perspective. For every story like yours (essentially winning the lottery) there are ten thousand people who swallowed their pride, ate shit at work with a smile, did whatever was asked of them, and got absolutely no financial reward for it at all.
My point stands, regardless of the outcome, though I can see how focusing on the money skewed the story for you.
If you find yourself later in life needing a professional assistance from that person who utterly does not value you, this is representing some much more gigantic failure and a worry about having burned a bridge is too insignificant by comparison to care.
Having said that, I don’t care so much about the “fuck you” aspect as much as, say, writing an honest Glassdoor review so that job applicants may avoid falling into their trap, or giving them exactly two weeks’ notice and no more despite whatever circumstances the project is in, and simply not staying in touch with them.
The moral here is not to endure abuse politely but to remain civil when possible. Leave before you get to "fuck you" territory. (Bonus moral: you think the startup world is crazy now? You should have seen it then.)