Sorry to bust your balls, but here's a thing from a fucking 6 years ago that debunks your B point to the ground.
30% has used to be standard in a PHYSICAL releases too, except these wont give you any convenience like cloud saves, remote play, family share, friend/community integration, support for older releases, huge ass discounts, linux support, and may other pro-consumer things. Valve takes this chunk and actually invests in the quality of their service.
Do you really think that MS or Epic would keep low markups if they have gained majority of the market?