This presumes open and direct confrontation, which means the relationship is going bad, and which I’d speculate most people would rather avoid. You’re forgetting a bunch of things, like what the investment terms might say about it, what your reputation is going to be down the road, all the ways people can apply lots and lots of pressure, how important networking can be, and the fact that in some cases the investor is providing other resources and/or might be the gateway to future investment. Getting to a place where you’re openly ignoring your investors and calling for a board vote to overturn them is a last resort, and is playing Russian roulette, not the first thing to try if there’s disagreement about their advice, right?
It doesn't require selling 50%. You can sell less than 50% of your company and still have the board majority controlled by investors, because those were the terms of the preferred investment.
If I'm not mistaken, YC takes a 7% at the seed stage, then the SAFE bumps it to around 15-20% on the next fundraise? From that, they're diluted down in subsequent rounds is what I presume.
Board control and share control are two different and competing things, with the board generally trumping shareholders.
The board is not controlled by shareholders. The board has a fiduciary responsibility to act in the best financial interest of the shareholders as a whole. But they (the board) essentially have full authority to decide what that means to them. Shareholders are effectively powerless outside of court, except for whatever power a board intentionally briefly concedes to them.
YC gets 7%, they have zero power to make you do anything.
Selling enough company that you loose 50% control is such a rare thing that it's not relevant to what most founders will experience.