This is an assumption and one that I believe is faulty. Humans' caring is often related to the degree of connection. Employees at a company are more closely connected to their customers than investors and are thus more likely to care more about those customers.
Monopolies are still bad either way, but I doubt that the failure modes at employee owned monoploes is the same as at outside investor owned monopolies.
Yeah. Saying "COOPs don't fundamentally change human behavior which has doses of greed" is like saying that a kingdom works the same way as a democracy since hey, it's all just humans. Simply involving more decision makers is a meaningful change. Certainly involving more decision makers that are outside of the business/legal/accounting class is a meaningful change.
Monopolies are still bad either way, but I doubt that the failure modes at employee owned monoploes is the same as at outside investor owned monopolies.