They're only thus bound if increasing stakeholder returns over time is an explicit condition of their contract (which I imagine is the case for executive contracts in many if not most public companies, though I don't know - can anyone confirm?)

But there's nothing in the (US, at least) legal definition of a fiduciary that proscribes increasing share holder profits by any means necessary. That's simply a convenient myth spread to normalize cutthroat, profit-seeking behavior at all levels of business.