You're the one arguing Valve might abuse their position to raise the cut. And you're asking me how do I know they don't ?
I'm not saying a big distributor never abused their position. I'm telling you Valve isn't.
You're skeptical, but based on what ? Tell me how much do you think each actors charge ?
(pro-tip, it's 30%. Even humble bundle takes 25%, 10 of them being for charity, leaving 75% for devs: https://support.humblebundle.com/hc/en-us/articles/202742080-Humble-Store-FAQ-For-Developers).
Heck, Sony even charges for the data: 16 cents per gigabyte:
http://www.gamasutra.com/view/news/113788/Sony_Charging_Publishers_For_PSN_Download_Volume.php
Dunno if they still do it though.
Couldn't abuse of position be more subtle, like failing to properly update the backend and rolling out features to consumers that have long since existed in other "online" platforms, but years after the fact?
I don't think monopolistic advantage is necessarily just a share of the game price upfront, but a relaxation on pushing the platform as a service forward to the most possible extent. You create margin where you can, not necessarily where it's most visible.
That being said, I don't know enough about Steam personally to truly comment on what inefficiencies or efficiencies exist in the platform due to its implicit monopoly, but from this thread, it sure seems like aspects of their service are decrepit enough to spur questions about its viability as a manager of uber-popular multiplayer games.
I "joined" PC because I wanted to be able to constantly push my gaming experience forward. Is that aligned with Steam? We're gonna find out!