Fiduciary duty is extremely rare to be the subject of a suit against a, let's say, CEO. It's a complex area of law because it isn't actually a law, nor specified anywhere, and not a requirement for corporate existence. So, it's a set of court decisions that future cases are built upon, but in general a house of cards in that it could be invalidated by a) legislation; and b) adverse rulings at any level of a suit.
It's a myth that the only purpose of executives is to maximize profit for the shareholders. It's a canard. PBCs are a counterfactual here, full stop.
https://en.wikipedia.org/wiki/Public-benefit_corporation
[note the significant use of "goal" in describing traditional corporations]