The big misunderstanding there is that while yes, if you use Steam hardware you're going to be using Linux, this won't apply to the hardware you already own that's running Windows, and this is what people are bitching about the most. They don't want to buy a new computer, they want their current fancy schmancy gaming rig to run something that's not Windows. With x86, the only option is Linux. But what GN is trying to do is to fit a square peg into a round hole. The reason SteamOS is this good is because it's tailored for Valve's hardware. It's not something that Valve could just copy over into a generic distro, or someone else could copy over from Valve's contributions. Even in this thread you have difference in opinions as to which distro should be used for gaming, because it'll always be a mess with Linux. Too many cooks spoiled the broth, but people want a perfect meal out of it.
In short: Linux will never be a viable drop-in replacement for Windows, and what Valve is doing with SteamOS is just making sure that you can pay for Steam games and play them on their hardware with their software with minimal issues. They're not trying to replace Windows, but every Dunning-Kruger idiot will run their mouth like they are.