You compromise a few influential people in the organization to get a foot in the door and ultimately your money - which you control. Your money will do the rest.
The 700 other employees who've signed on have agency. They can refuse to go to Microsoft, and Microsoft wouldn't be able to do anything about it. Microsoft's money isn't some magical compelling force that makes everyone do what they want, otherwise they could have used it on the board in the first place.
Money changes people. Especially when it’s a lot of money. They got used to the money and they want it to keep flowing - the charter be damned. Everyone has a price.
Everyone has a price, yet Microsoft can't buy the three board members on OpenAI board. Curious.
Your initial statement was flatly wrong, and you're grasping at straws to make it still true. Microsoft wouldn't be able to get anywhere if the people who work for OpenAI chose to stay. The choice they're making to leave is still their choice, made of their own volition.