Regardless of whether or not it was a "mistake" (I don't think it was... OpenAI is so far ahead of the competition that it's not even funny), the fact remains that a) Microsoft has dumped in tons of money that they want to get back and b) Microsoft has a tremendous amount of clout, in that they're providing the compute power that runs the whole shebang.

While I'm not privy to the contracts that were signed, what happens if Nadella sends a note to the OpenAI board that reads, roughly, "Bring back Altman or I'm gonna turn the lights off"?

Nadella is probably fairly pissed off to begin with. I can't imagine he appreciates being blindsided like this.