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