I asked myself the same question with the Xbox 360 back in 2008 when I started building PCs but had already bought an Xbox 360 in 2007.
Once I started playing the same games I had bought the Xbox for (Bioshock, Orange Box, Tomb Raider Legend/Anniversary, Mass Effect which were Xbox console exclusives at the time) on PC through Steam, that was the point where I realized my only reason to keep the Xbox plugged in was for HD-DVDs (back in 07-08 before BD killed it) and the tiny handful of MS Studios exclusives, namely Gears and Halo. I had no need for an XBL Gold subscription given the minimal use it saw and because the Xbox is hobbled without one, I simply used it even less.
I'm still curious to see what the Xbox 3 will offer beyond Kinect 2 though and there's a fair chance I'll end up buying one at some point in its product cycle, but probably not within the first year or two. If it's really nothing new beyond more of the same but improved, I may well end up not supporting the Xbox platform in the future at all.
It will probably boil down to which games MS deliberately keeps exclusive to the Xbox platform, which is essentially the only reason I still use (sporadically) the Xbox 360 today.
But as for a Win8 PC being my replacement "console" if I keep it plugged into the living room HDTV, I'm unsold. The Win7 PC I currently have plugged into the HDTV never gets used because it's strictly for PC games and it's easier and faster to pick up a controller, push a button and start playing without having to pick up the mouse and keyboard, boot up, open Steam, enter my Steam password, wait for updates, etc. Maybe if it was on 24/7, but I don't PC game enough to waste power like that. I tried a Windows Media remote for doing the PC start up from across the room, but that's really only for watching movies as you still have to pick up the keyboard and mouse to set everything up before switching back to your remote and game controller.
For me, PC gaming is still best experienced at the desk where my workstation is set up.