Everything capable of computing and is owned by a person is a PC: macs are PCs, the PS4 is a PC, smartphones and tablets are PCs, even my brand new Panasonic smart rice cooker is a PC. What people call "pc gaming" is nothing more than windows gaming. Windows games only work on windows/x86 machines(at least out of the box). Steam Machines are not an example of Valve trying to save windows gaming.
IMO, valve is instead trying to create a new version of "pc gaming", in the shape of an open home console(as opposed to the sony/nintendo model closed model) while also trying to expand in the next hot market: smart TVs/living rooms. Having it's own software and hardware platform where your service is the default is also a great way to reduce the visibility of rival game appstores like GOG, Origin and non steam popular games(Minecraft, LoL, Blizzard games).
Not only that but Valve is trying to save something, this something is itself. The business may look great nowadays, but it's foolish to think they're invincible. Windows and Mac are becoming walled gardens, not very friendly towards apps outside the official app stores. Windows PC sales are in record decline. 65 million steam accounts may look impressive at first glance but considering that steam is a FREE service and that even the PS3, the overpriced console that sold the least the last generation, still managed to grab 80 million users(let alone way over a hundred million PSN accounts), it's clear that Valve doesn't have as close as many users as it could. If Valve lose it's momentum, they could easily become irrelevant.
On the other hand as long as Actvision/Blizzard, Minecraft, EA and LoL (and in Japan, porn VNs) exist, Windows PC gaming will exist. Contrary to popular internet forum belief, Windows PC gaming is much more than Steam. I personally believe that, if wasn't for the crazy seasonal sales and mandatory steamworks in some games(Civ5 in my case), many people(including myself) wouldn't even bother with the service.