Others have commented on how Microsoft actually has access to the IP, so the odds that they could pack their toys and rebuild OpenAI 2.0 somewhere else with what they've learned, their near infinite capital and not have to deal with the non-profit shenanigans are meaningful.
I'm not saying Sam is needed to make OpenAI what it is, but he's definitely "the investors' guy" in the organization, based on what has surfaced over the last 24 hours. Those investors would rather have him there over someone else, hence the pressure to put him back. It doesn't matter whether you and I think he's the man for the job -- what matters is whether investors think they are.
TL;DR the board thinks they have leverage, but as it turns out, they don't
Honest question, do you have a source for that? Is it conceivable that Microsoft has some clause that grants them direct access to IP if OpenAI does not meet certain requirements. It is difficult to believe that Microsoft handed over $10B without any safeguards in place. Surely they did their due diligence on OpenAI's corporate structure.
Microsoft has a lot of experience interacting with small companies, including in situations like this one where the small company implodes. The people there know how to protect Microsoft's interests in such scenarios, and they definitely are aware that such things can happen.