I don't see any reason why Linux couldn't be mainstream, it would just need a large enough company behind it to break up the Wintel monopoly (which, to be fair, is already threatening to slightly unravel.)
I think Android's proven that consumers don't really give a fuck what they're using, it's all down to the big players coming to an agreement to use something common so app developers can give it critical mass. With desktop PC's that was Windows, and all that momentum isn't going to just disappear. It could have easily been Linux back then if it was around, and more importantly, if Gates wasn't the smartest business motherfucker on the planet.
If Steambox is a success we could see the first instance of a traditional Linux distro being successful for consumers, but that's narrowing the scope quite a bit. Linux is already successful in many other areas (servers, smartphones, etc.), but the fabled "consumer desktop OS" has eluded it due to market effects that began in the 80s and 90s.