I think you forget you're posting on YC's forum. YC literally does this (with much higher stakes, by the way). Obviously, the great majority of YC companies don't end up being unicorns, but every now and then you get a Dropbox.
And to address your first point, Steam's SDK (for anyone that's worked with it) obviously sucks. But Valve can afford to release a crappy SDK because they're the big player and they don't care. Obviously, if I made a game distribution platform, the back-end would be minimal and packaging would be programmatic. E.g.: upload your binary, we'll package it and deploy. You don't need half the crap Valve peddles anyway (Steam overlay, chat, etc.)