Hmmm, mod points or comment..... Oh well.
"The answer is: No...
As I see it, the Steam eco-system will be no different than the current consoles (XBox, PS4). The Steam boxes will have the advantage over the consoles of higher-end graphics, game controllers, etc. Windows boxes not only support higher-end gaming but also a wide variety of applications. A gaming rig can, and usually is, also used for gaming, photo editing, finances, and many other applications. Much like tablets, it's all about the apps...
Don't get me wrong, I'm happy to see the expansion of gaming. However, Steam is not the savior of the Linux desktop. In my opinion, It will be just another console..."
At some point the market will decide there are too many choices. This happened back in the 1980s when there were loads of different types of home computer and eventually they got thinned out through the 90's until Windows PCs were basically it. Valve is likely to turn console gaming on its head if they can get a sufficiently console like experience in place (Big Picture is close but enough games don't work with it that I still have to keep a keyboard and mouse close at hand) and enough AAA games come to it. Certainly, the market is saturated with PS4/XBone and then SteamBox and of the three I wish the SteamBox most success because it already has a massive game library compared to the other two, plus you can upgrade your hardware and keep your games. That's a killer improvement.
As for Windows, the main reason people use it is applications. When I talk to people about why they don't use Linux or Mac on the desktop there are largely viable replacements for the apps they use (especially true with the Mac) but games always come up as the main reason. Take that away and Windows is severely weakened in the market. Sure, it will hang on for a long time especially in corporate environments but these days home users have very little need for a PC when a tablet can do their social communications and a games console can do the gaming. I have a Mac and a PC (Win 7) at home and I barely use either because I have to actually go over and sit in front of them. Sure, I use my work Mac in the office all day but in my off time I don't want to sit at a desk. Steam on any computer is OK but it still can't be totally driven without a keyboard and mouse so maybe Valve's new controller can fix that (I hope so) in which case I'll leave my Windows box in Big Picture mode all the time and enjoy my games from the couch in full 1080p or more when I next upgrade and I won't lose my library. With SteamOS taking on more of the games I already own, a wipe to that is in the future for my PC and I'll keep the Mac for the boring stuff like work.
MS is desperately trying to remain relevant but they're bouncing around taking shots at everyone in sight because all these little devices are pulling the eyes away from their platform. Windows 8 has done little to improve things because it looks and works so badly unless you tweak the hell out of it with Classic Shell to get rid of the nightmare modern interface and restore it to something that looks and behaves more like Windows used to.
Sitting here at my Mac I have VMs for many different Linux distros, various versions of Windows too but I run OS X because I can run everything on it either native or via some form of emulation. Games aren't really the Mac's forte but that's OK as I don't want to sit in front of a keyboard to play games and I want a viable replacement for my current Xbox 360 (which I dislike more with every update) and MS just burned the Xbox platform by releasing the One without any backwards compatibility. Similarly, Sony's PS4 has no library and the price of games have gone up another 15-20% over the already outrageous prices so no sale there either as I can't pick up cheap back catalogue stuff to fill out the collection. Pity, I had hoped to play many of the PS3 exclusives and as it stands I'll likely buy a PS3 cheap at some point soonish to do that. I also just bought a WiiU because Nintendo is still innovating and it plays my current Wii games so we already have stuff to play along with the couple of WiiU games we got and you can pop them off the TV onto the controller screen. That's cool. PS4 and Xbox One? No back catalogue, expensive games, sub-PC graphics and all that lovely DRM. Nope, don't think so.