Non-expert opinion
IMHO Microsoft has had an outsized influence on OpenAI's leadership and mission by providing substantial support for Altman. If they had not provided that support, Altman would have remained out, and Toner and others would have stayed on the original OpenAI board, in support of the company's original mission. The 700 or so folks who signed a letter to reinstate Altman may not have done so if Microsoft hadn't shown unwavering support for him (or maybe there was some other outside influence at play?), but, if they had resigned as a result, and the company had subsequently folded, it would have done so in fulfilment of its missiion that it is better to fold than to push "dangerous" AI out onto the world. The company's safeguards were working, but were meddled with by Microsoft's influence.
(I know nothing about antitrust rules though)