I’ve got some experience in not for profit boards and for profit boards. For profit boards serve shareholders. Not for profit boards are often the ultimate authority. There are no shareholders. They may have members who fill that role, or they may not. They can also be essentially self-sustaining, with only board members electing new board members.
Unless given some authority by contract Microsoft likely has no ability to do anything with their board.
Also, "hiring the best lawyers in the world" is similar to the "nine women can't give birth to one baby in one month", that is, from a certain point on both sides can "hire the best lawyers in the world", or pretty much close to it, an order of magnitude (or two or three, like MS has over whoever will stand behind the OpenAI's board members in helping them with money) bigger scale doesn't necessarily guarantee the expected outcome.
If one company could sue another smaller company out of business at will, there would only be one company.
It does happen that companies go bankrupt fighting invalid lawsuits, no doubt, but it’s rare and it’s almost never a company that just built the most talked about product of the decade and raised an 11 digit war chest.
It will be hilarious if they sold all their IP to Google or Amazon.
But it's more likely in such a case, they will just open source / toss into public domain all their IP, shut their doors, and start a new non-profit which MS has no good reason to sue.
Or that if Microsoft did sue (perhaps because something that happens in the near future violates their contracts) OpenAi just spends like 1% of their cash on legal fees. $100 million in legal fees is not an existential risk to companies who recently raised $10b.
That just isn’t how the legal system works. You can sue a small business into bankruptcy because a few million in legal fees is enough.