Isn't this idea that corporations should not buy back their stock a consequence of the idea that company shares are for speculation, not to get the company operating capital?

What about a company that sells shares, builds a business, makes a profit, and fulfills their mission? Can't such a company then return some or all its value to its investors by redeeming--buying back--its shares?

That's what happens when a company is sold. VC investors expect it.

Another model of company responsibility is to account for externalities. For example, if a company pays such poor wages that workers need public aid (SNAP/"Food Stamps") to feed their families, that's an unaccounted externality. The company's profit in that case comes at a cost to the government's human services operations.

Similar arguments apply to exhaustion of natural resources, and to pollution.

All that being said, the speculative framework for company valuation is certainly the entrenched model. All big companies are basically financial entities with a side hustle in some kind of product or other.

For what it's worth, Berkshire Hathaway (Warren Buffet and colleagues) rescued General Electric on the condition that they definancialize and stop treating their products as a side hustle.