Sadly I expect the vast majority of shareholders would individually trade the future health and profits of the company for their own immediate benefit as well, were they in a position to make the same choice. You're right that since they collectively don't benefit from such short-sighted focus, they wouldn't make the same choice.
It's outside my bailiwick and I'm not quite sure how it happened, but it seems to me that over the course of a few decades (70s to 00s?) we went from a model of corporate management where the various mechanisms of "cripple the company for the short-term benefit of upper management (plus a few well connected others)" were neither sophisticated nor, well, thinkable, or at least not acceptable, to one where both the ability and the practice of doing so are near-ubiquitous.