They would all leave Steam? Even though even with the royalties they get more per game sold than for a boxed version? I don't think so.
It's not unlikely 3rd parties will shop around, there will be competitions once download market is big enough. Sony, MS and Nintendo have their own. Amazon, Google, Apple and the like will join in eventually.
Because a normal PC doesn't act like a console, the fixed hardware spec is not an option and neither are the cut throat margins ... if Dell or Alienware were willing to build to spec they could be allowed to make their versions, but building boxes to specs with only cosmetic differences and small margins isn't going to be very attractive to them. If you want something done right you have to do it yourself first, you have to set the standard.
I am sure the Valve crowds that play Half-life can install a $150 video card into their PC and play games. Anyone that care enough on the graphical difference between PS3/360 and Valve console are most likely know how to intall a video card. Valve is going to have a hard time differentiating it from other consoles and ordinary PCs.
I don't think there is any other standard needed for PC gaming other than DirectX and Windows standard. What PC needs is a better copy protection and a standard wireless gaming controller on top of keyboard and mouse. Probably need standard motion control too. MS have started that, though they don't promote it as hard as 360. But PC is still a Microsoft platform. And if your console is going to be a PC and uses Windows, MS will define the standard for you. The crowd that this console is targeted will know that.
If we assume 50$ for Windows (Windows Tax pricing) and 100$ for Fusion it should be doable without selling below cost.
What's set consoles and PC is not the OS or the CPU/GPU. It's the marketing behind them. Valve would need to match the marketing budget of Nintendo, Sony and MS. It's no problem if they managed to sell 30+ million consoles, but if they failed the marketing cost is going to hit them hard. On top of that the setup cost and the like will hurt them if they failed. To me these risks are too great for too little gain.
Furthermore Valve games output are very far in between. Sure they make great games, but I don't think they can rely solely on themselves for exclusives. So they have to go hunting for great exclusives and possibly need to outbid the other three. And we know these exclusives aren't a sure thing either. They can flop.
