I actually also believe that Google Play and Apple App Store as marketplaces also provide "enough value" to potentially justify a fairly high price tag, but in their case it's not actually fair because the problem is that nobody else is even allowed to try to provide similar value at any cost. For example, both Google and Apple provide "free" push notification infrastructure, which is sort of necessary: if everyone was running push notification infrastructure, it would be pretty bad for battery life. However, the net effect is that you're being forced to price all of the value that they provide as platforms, into their marketplace, whether or not your app needs or wants their "value", and that's the problem. This doesn't quite compare to the situation with Steam, and I think that warrants more recognition.
Frankly, Steam sucked ass when it first came around. It was relentlessly mocked, and the only reason people tolerated it was because you needed it to play Half Life 2. But... they never stopped improving it. And frankly, even if this makes Valve a "worse" company from a position of investors and onlookers, it has made Valve a better company for consumers to be able to trust. It's pretty obvious that not every product or service Valve puts out is fantastic, but in the same token that things which are easy at "normal" companies are impossible at Valve, things that are impossible at normal companies appear to be possible at Valve.
I hate to romanticize it too much, but I'm not even a huge gamer, and I still feel like Valve has done very well by most of their consumers and developers. If anything, the biggest trouble they seem to have is deciding how exactly to moderate/censor the Steam store, given all of the different external pressures. Now that is a tough problem and they've had a tough time figuring it out.