Not to be pessimistic, but don't overestimate your leverage. You may know that you leaving would be a big setback, but the founder may not. Careful with the ultimatum approach too. Even if the founder gives in, it may create resentment if your conversation ends up being "if you don't do this, I'll leave, and everybody else will follow!"
It's interesting that you think that a very small company is more likely to implement this new policy. I would have said that a bigger, more stable company would be a better candidate. Founders at small companies are betting everything on their shares, they will be more reluctant to giving them away too easily.
Agreed, I don't want the conversation to steer into an ultimatum nor using that line of reasoning. That will leave a sour taste in everyone's mouth.
I think the conversation can be approached in a more positive light and framed as ensuring all the hard work and long hours put in by me and other employees is fairly compensated and we get to retain the equity that vested.
As one of the key employees I have leverage. Am I irreplaceable? No, but it would definitely be a setback. One that the founder cannot afford at this time.
I can't speak for the other employees but I imagine they won't be happy either once they learn how the founder treats employees.
Besides appeasing existing employees there is the added benefit of attracting and retaining talent going forward. This is a great point that you mentioned and shouldn't be overlooked especially considering how competitive the valley is for talent.