They don't have to make the best decision to advance the interests of the shareholders. It is impossible to know if an option that wasn't taken would have been better. They're require to act in a reasonable way that fits market norms and the data they had at the time. The decision of whether to accept Musk's proposal, regardless of how much he claims it to be the "best thing to do", is very subjective and complex. As long as they don't rely on immaterial facts (such as their dislike of Musk), it's impossible to prove that they made the wrong decision because we can't A/B test reality. They might still believe that Twitter can be worth $60B in a year and if they have enough data to support that, they can do whatever they think is right.
