You are making some strong claims when this is very dependent on the exact scenario. My bet would be that you don't want to be replaceable because if you don't

You are making some strong claims when this is very dependent on the exact scenario. My bet would be that you don't want to be replaceable because if you don't hold out to the IPO, the startup (and any new partnership they make) have every incentive to claw back as much money as possible. They could do this even if you stay but when you stay you need to be assertive enough to defend your position. I don't know how well everybody does at asserting their position from a technical role when you aren't in the room where the decision making happens. The only way I'd ever work at a startup with the goal of making a bunch of money based off of those shares is if I absolutely knew they couldn't and wouldn't fuck me over. I have no faith in any person in this world apart from very few. I would rather "grind" for the money i.e. work 8hr days and make it to my money goals quicker than 99% of the outcomes of choosing the startup path. That said, you can get good experience but the most important experience you get is knowing where you stand in any organization.