The one thing I could see that might miraculously save their relevancy within the company - aside from a complete windfall in terms of X-Box One sales and software/service attach rates - would, ironically, be Valve. Not so much the "threat" the Steam Machines pose to the console space, as I'm still not convinced many console-only gamers are going to make the jump, but rather the threat the entire initiative represents to Windows as a preferred PC gaming platform.
The fact that PC gaming enthusiasts are Windows users by default is something Microsoft has long taken for granted. There's a lot of value there, even beyond the guaranteed users: even if you were considering using another OS, you would likely reconsider because of PC enthusiasts telling you that a lot of software - most of it games - don't run on a competing OS. A major part of this was that they weren't especially worried about it; Windows was so deeply ingrained as the de facto operating system that they never felt a particular need to consolidate their strength. There wasn't any need to worry about Windows having especially strong market penetration with any given demographic, because Windows was ubiquitous across every demographic.
Now, though, they're getting to the point where they have to accept competitors offerings as legitimate threats to their market share. Apple's drawn an increasing number of people into their ecosystem through the iPod, iPhone, and iPad, and in particular is making strides with younger users. The value of China as a market means that simply pointing and laughing at their state-sponsored OS is less and less of an option moving forward, as the nation's digital renaissance rapidly creates a population of non-Windows users of incredible size. And now Valve is trying to tempt PC gamers - the one demographic Microsoft was absolutely sure could never leave them - into escaping their proprietary hold.
If I were at Microsoft, I might be looking at SteamOS and thinking to myself, "There's no way this will get enough users supporting it to become a viable software platform unto itself - not unless those Steam Machines sell a surprising amount." That thought rattling around in my head, I might see further investment into the X-Box One as a useful catspaw with which to try and stymie the success of the Steam Machines so that SteamOS fails to gain traction and there's no significant threat of Windows losing its status as the "only way" to play PC games.
Though, if I were to guess, Microsoft's brass probably don't actually have a firm grasp on exactly how valuable being, for all intents and purposes, the "exclusive" OS for the vast majority of computer games is. There's no memory there of when they weren't, after all, so the market effects are impossible to accurately estimate. I don't think they ever took much active interest in Steam, after all, and Games for Windows Live was a confused and perfunctory effort at best. They probably think that once they get their half-assed app store up and running everything will be sunshine and rainbows forever.