Saying its 30% of gross income for advertising is pretty disingenuous. They also handle the distribution and payment systems which is very much not insignificant along with other more insignificant things.

I have serious issues with some aspects of Steam,and if its creating billionaires theres obviously a problem. But taking a cut of game sales isnt those issues. We can argue about the exact % that would be reasonable, bc some amount definitely is. But thats not the actual issues with the platform.