I don’t get wolfire’s point here. Yes steam takes a hefty 30% cut but game developers are free to sell directly if they want to. Unlike apple who have completely locked down the iOS app ecosystem or Google who allow sideloading but scares and warns people against downloading apps from non Play Store sources, steam does nothing to hinder games not sold through it. If there was a competitor who was as good as steam but took a smaller cut, then that competitor would have been the market leader in place of steam.
Also the main point is valve does not do anything to prevent you from selling the game directly or through other stores.