The main mistake the board made was tactical, not philosophical. From the outside, it's seems likely that Altman was running OpenAI so as to maximize the value of the for-profit entity, rather than achieve the non-profit's goals, if only because that's what he's used to doing as a tech entrepreneur. Looking at OpenAI from the outside, can you honestly say that they are acting like a non-profit in the slightest? It's perfectly believable that Altman was not working to further the non-profit's mission.
Where the board messed up is that the underestimated the need to propagandize and prepare before acting against Altman. The focus is not on how Altman did or did not turn the company away from its non-profit mission but instead on how the board was unfair and capricious towards Altman. Though this was somewhat predictable, the extent of Altman's support and personality cult is surprising to me, and is perhaps emblematic on how badly the board screwed up from an optics perspective. There were seemingly few attempts to put pressure on Altman's priorities or to emphasize the non-profit nature of the company, and the justification afterwards was unprepared and insufficient.
From the outside though, I don't understand why so many are clamoring to leave their highly paid jobs working at a non-profit who's goal is to serve humanity and to become a cog in a machine aimed at maximizing Microsoft shareholders' wealth, in defense of a singular CEO with little technical AI background who's motivations are unclear.