If the cut doesn't provide value for me on the part of Valve, devs will create this value through other means that I care more about: video games.
The amount of dollars that is redistributed to create games, jobs and opportunities will be more valuable and will be better for everyone involved from consumers to the people working in the industry. The social value is much higher through redistribution than centered around one company.
Valve uses their money to develop and provide all kinds of tools for developers RIGHT NOW. Tools and technologies that make it easier to make, sell, and maintain games. There's no way that doesn't leave two more, better games for me to choose from. And again, to emphasize, that is happening right now. It's been happening.
Also, they forfeit a lot of their cut so that resellers and small publishers/developers can do business independently, and compete on prices. Prices lower than MSRP.
These are tangible, specific benefits right now. the way you're describing things getting better to you with a lower cut, is beyond vague and I can see will never be measurable.