I don't think Steam is rent-seeking because:

• no cost to maintaining an account
• no cost for keys if you sell stuff outside the Steam store
• no cost for downloads
• no cost for improvements to games

Valve's customers are publishers and devs, and they're charging a finder's fee for connecting customers to the games. To me, that's not rent seeking, that's a direct exchange of money for a service. If you don't think the service is valuable or think you can do better, then generate keys and sell them elsewhere and you won't need to pay Valve a cut.

Valve is capitalism done right imo. You only pay when you receive a service, and only when you profit from the service. Steam also has a fantastic refund policy as well, which is surprisingly rare in the digital goods market.