Gabe wants to get away from Windows because it is competition for his digital distribution software. If the Windows App Store offers developers / publishers a better deal than Steam, why wouldn't they go with it? Steam has the users now and can afford to move off Windows, and away from the competition, but if you actually think this is FOR GAMERS, you're not particularly smart.
This much is clear: Valve (or, specifically, Gabe) doesn't like Windows 8. You almost wonder if his complaints about it were half-justified statements to talk people out of wanting it, rather than true, fundamental flaws in it. If he can sully the name of Windows, he can talk up his stuff as an alternative.
But while Steam has the users to create an OS, I don't think you can go so far as to say it has enough users to compete with Windows. For starters, Valve isn't Microsoft, so it's not going to be able to throw as much money around to draw developers--at least, not like Microsoft did with Gears of War or Titanfall.
The only way that "enough users" theory could be tested is if Valve took Steam away from Windows. As long as Steam is on Windows, they don't have enough users to support SteamOS. You can get more out of Steam on Windows than SteamOS, because of the sheer volume of games that aren't on SteamOS. They'll have to get something like 90% of games on SteamOS, then pull the rug out from under Windows to get people interested. That, and they have to pull it off without letting Microsoft get a competing alternative out there.
Windows is familiar. It is versatile. It is proven. SteamOS is basically a game console OS, in terms of its market. I don't see how you're going to talk people out of PCs for that. If Valve ever DID take Steam out of Windows, you might be able to talk people into SteamOS-based machines to supplement a PC and replace a gaming console, but I don't see SteamOS as something that is targeted at, or capable of competing with, Windows.
