“They could take more and still control the market” is a confession, not a counterargument.
And it directly contradicts saying they need that much money, in order to… “provide value.” An aggressive hand-wave that ignores how Sony and Microsoft take the same cut for platforms they own and control completely.
This 30% off-the-top is a de facto standard that’s basically just left over from when Nintendo had 90% market share and had to physically manufacture cartridges months in advance. I don’t care what Valve says they’re providing - they did not do half as much per game as the people who made the fucking game. They don’t deserve a third of their money.