If Valve bumped their cut from 30% to 40%, do you imagine publishers would rush to EGS? Epic's cut is already 15 points lower than Valve's. It hasn't moved the needle.
Valve kills studios by saying 'no thank you.' They have immense power. They just don't use it in any way that freaks people out. The mere possibility shapes the entire industry. Only niche studios try weird shit, because large studios don't risk poking the bear. Games want to feature nudity and intimacy, but most are so self-censored, they could be televised. The cultural prevalence of nude mods is proof of demand that has been frustrated.
If you'd rather blame Mastercard and Visa openly dictating what art can and can't be sold, by all means, we can talk about their joint control of online payment. But it might get blunt if you insist one store taking Bitcoin means that's not a duopoly.