The board of OpenAI has their hands on the ultimate self-destruct button, though. If M$ made a power play, worse case scenario they end up with ownership of the for-profit OpenAI, but they'd still have no control of the non-profit, which owns the intellectual property.
If the deal goes sideways, the board of OpenAI (the nonprofit) could just dump everything onto the open internet. All M$ has is a substantial but minority stake in a company that the non-profit OpenAI owns all the beef of.