You know what also justifies Valve's 30% cut? Their outstanding efforts in getting games to run on Linux, and the overall impact that this had on the Linux community.
I don't think I'd be running Linux as my only daily driver if not for this. I was slightly dreading switching because I feared spending hours trying to fix broken games, but it's been astonishingly straightforward (which facilitated me learning to live in Linux in a way I hadn't been able to when was dual booting with Windows)
For most developers, that's not much of a value. The Linux share of the gaming market only exists because of Steam. 99% of those gamers would just play on Windows if Valve hadn't pu in the effort.