First, from the point of view of this humble console only gamer, I don't see how these announcements change anything as far as the coming console generation goes. Neither the mantle or SteamOS announcements have made PC gaming suddenly look more appealing to me because you'll still have to upgrade more often (then consoles) to get the best performance. GTA V is a game that looks light years ahead of the first games released on the 360 8 years ago and my 360 runs GTA V like a champ. I doubt mantle will pull off the same for 8 year old PCs, not with out some hardware upgrades. Maybe I'm misunderstanding what you are saying but I just don't see this challenging consoles in the optimization department... consoles are always going to be king here unless every PC gamer buys the exact same hardware at the same time.
Oh, I'm definitely jumping to conclusions. But I'm not saying it's a foregone conclusion, just noticing which way the winds are blowing. Let me ask you this....is your launch xbox still running? I'm sure you've had to buy a new xbox at some point. Even if it's a slim, it's still the same hardware inside. And since 2005, how many times have you upgraded your smartphone? Probably quite a few...and each one has been better and faster than the last. Even if mantle only gets them halfway towards being as optimized as the consoles, and even if SteamOS is still a little trickier to use than an Xbox....at some point the gains from the newer, better hardware and the open platform with more competition will start to outweigh the advantages of a standardized platform. We're seeing that happen with Android vs. iPhone - there's only one iPhone, and that tight platform has it's advantages. But if that latest iPhone isn't for you, there's a zillion android phones out there that all run the same OS and apps...and now you've got choice and performance that you never had when sticking with just Apple.
From the point of view of this console only gamer, I just don't see these announcements changing the scene a whole lot, at least not for many years. Still looks like Sony vs MS to me, nothing in the past few days has made me question my PS4 pre-order out of the hopes for something better coming soon.
Honestly, I don't even get the excitement around SteamOS and the Steam Machines. I guess if you are already a Steam user and PC gamer it is exciting but, it didn't do anything to make PC gaming appear like a better solution the then PS4 or Xbox One, not to my eyes at least.
Yeah, I'm not saying this changes everything overnight, but the seeds are being planted for a revolution. It still needs time to grow. For the next few years Sony and MS run the show, but they no longer seem as invincible as they used to be. This isn't some kickstarter like ouya, Valve is a big player in this space that isn't toying around. I'm still buying a PS4 and Xbox One on day one, but if they never iterate on hardware, and they're the exact same internals 3-4 years from now....they'll probably just start collecting dust, because I'm not going to be interested in playing games in sub-1080p/30fps for very long. Not when someone else is putting out vastly superior hardware with a decent couch UI for a similar price. Valve's got the right concept, now they need to execute.