Of course it would be an OS built for Steamboxes. They aren't going to ship Ubuntu, they are going to just change a few things and throw a brand name on it. Will all the PC functionality be preserved? Yes. You'll be able to do all the things you do on the PC, in basically the same way, it'll just have a different name and style that fits the Steam brand. And of course you'll have the option of installing a different OS while preserving Steam functionality.
I know you want to pretend that makes it a console, but again, that is just a false description. Calling the PS4 a console is just a market division. It is a PC with a modified operating system intended to limit functionality to Sony approved software. Whereas a Steambox is nothing at all like that, and it is dishonest to say it is.
Linux enhances it because Linux, with proper corporate support, is about a thousand times better than Windows. DirectX is garbage. Windows is garbage. All Linux problems stem from lack of commercialization - now that it has been successfully commercialized with Android, and will be again with Steamboxes, that corporate support will be forthcoming.
I mean you can't sit here and say it is Linux's fault that companies don't build drivers for it, than insist that it would be a failure even when companies do build specific drivers for it, as Android phones do, as Sony did with FreeBSD, and as Steam will do with Steamboxes. Especially since Steam will likely keep their developments more or less open source, instead of just stealing other people's work and calling it Orbis OS.