These mutual funds and ETFs actively participate in corporate governance, and the overlapping ownership of most stocks in the market is a very underrated problem.

Economists have found evidence that as ownership concentration increases, firms act more like a monopoly.

See this paper for one example in the airline industry: https://onlinelibrary.wiley.com/doi/full/10.1111/jofi.12698

It seems relevant - if company leadership is attempting to act in the best interests of the shareholders, the fact that the shareholders also own their competitors would tend to bias them against engaging in a price war (even one that they would have a good shot at winning).

there’s an ongoing debate about whether this situation actually does pose an antitrust problem across many industries.

it’s at least not totally implausible.