You’re thinking about this narrowly. The broader implication is rising income inequality. It’s not sustainable. If GINI gets too high it can only end in reform, revolution, or anarchy.
Who gets to decide what the gap can be? Employees? I don't believe there are ever "unbiased, third parties" in these situations. You want to pre-emptively prevent one outcome by controlling the company - but this level of power also comes with side effects. Why take the risk of starting a company when you don't control income? Who's to say people won't want to continually lower that income gap until countries stop innovating? You have to consider the negative side effects of your own solution...else risk thinking about the problem "too narrowly".
Co-ops are one option to be sure. There are many successful co-ops, though usually in industries that don’t require huge startup capital.
The idea that billionaire returns are required for motivation is laughable. 99% of people are similarly motivated by a $10M return as a $1B return. The point is not necessarily to rid ourselves of capitalism entirely (though that’s logically the optimal outcome in the long run, since by definition capitalism’s optimization function is independent from socially or even individually desired outcomes), but rather to bring compensation disparity down to a reasonable level. I’d recommend targeting a GINI of .3 to start, if you want to put a number on things.
