It's not reductionist at all; that's exactly how it is in a purely capitalist system, and how it is in America. It's different in other countries, like European ones and Japan, where companies really do have a responsibility to employees and society at large, but in America they don't have any such responsibility, no matter how much you would like it to be otherwise.
This is clearly, obviously not the case. There are many ways to make money and the management of a company has a lot of freedom to choose how to go about it. Different companies have quite different strategies and cultures. Some treat their employees much better than others. Pretending they're all the same is sort of like saying it doesn't matter who wins in an election.
What we can say is that when new management takes over, sometimes things change quite drastically. But this isn't deterministic and so you can't conclude all that much just from knowing the theory of shareholder value.
Yes, but you can find clear trends when you look at different groups, such as American corporations vs. European ones, and if you do that, you can see that employees, on average, are treated much worse at American companies. While there are some success stories in America, such as Costco (which is pretty famous for paying and treating employees very well, considering the jobs there are generally low-skill and don't require college degrees), the American system rewards companies that don't treat employees well and they tend to do better when competing with companies that do.
Executives may often have excessive power and influence, sure. And there is much room for improving how the system affects all individuals. Is this what you are trying to imply?
You can disagree all you want, but executives actually run the corporations, and their boards of directors (stacked with other executives who are buddies of theirs) don't care what you think, and will continue to run these companies for the benefit of the executives rather than the shareholders.