People that aren't familiair with linux won't buy it either.
People buy Android that aren't familiar with Linux. Any user-friendly GUI you put on top of Linux requires nothing more than common sense (just like Windows). Gnome, KDE, Xfce, Cinnamon, Unity are all just read, point, click.
Steambox may not take off like the other consoles, but it is a good step for Linux gaming. A main reason gaming companies don't make games for Linux is because it isn't stable in terms of compatibility. What works on one kernel might not work on another, that is why having 100 distributions is a pain to support for companies and they don't even want to venture down that road. It is also why Valve only officially supports Ubuntu.
Having a Development platform that is locked down where one company has control over the OS and game makers work with that company on compatibility will lessen the burden of game development on Linux.