It could probably be argued that the board didn’t do what was best for the investors, which is what they exist to do.

Incorrect. OpenAI LLC (the traded company) does not have a board of directors. The board of directors actually belong to the parent company, simply “OpenAI”, which is a nonprofit organization – the only thing that they’re “beholden” to is the OpenAI company charter document.

Here’s a simplified breakdown:

Board of Directors =[controls]=> OpenAI (non-profit) =[majority shareholder]=> OpenAI LLC =[employs]=> OpenAI CEO

OpenAI LLC is obligated to act in the best financial interest of their shareholders, but OpenAI LLC does not actually have control over who sits in the CEO chair. That power goes to the majority shareholder, which is the non-profit “OpenAI” parent company – a company beholden to their board, not shareholders.