The thing is - a company is only responsive to those that own shares in THAT company - and they may own nothing else. If you start worrying about shares of other companies that may be affected by the actions of that company, you get into impossible to solve conflicts of interests.

Not all shareholders have the same portfolio, nor the same time horizons. If what that company does has differnet impacts on different other companies, how do you decide between these effects (including that on the shares of the company itself) which is most worthy.

I do think that the solution I mentioned (conglomerates, and internalisation of these wider externalities) are relevant in that context.

In the long run, we're all dead. John Maynard Keynes