The biggest console niche for me is that it's easy to throw one in my backpack and take it with me. I spend like 3-4 days a week between a few houses, and taking a PC with me would be pretty annoying, especially on TVs that don't have HDMI for easy access. I know you can get smaller PCs, but then you sacrifice a lot, I think. In addition to that, most houses don't have a good setup for a mouse and keyboard in their living room. It's usually like 2 couches in front of a TV, and no place for a KB/mouse. Most people don't have PCs hooked up to their TV, it's sitting in a corner on a desk somewhere. My situation is a bit different than most, but it's a compelling reason for me to keep using consoles some of the time, and having my dedicated PC at home. I could get a laptop capable of playing the games I want on PC, but thats probably going to run $700-1200 bucks compared to dirt cheap consoles. I can snag a $79 wii, $50 ps2, $120 xbox360 and feel like I can abuse the hell out of it: Cheaper than an operating system alone for a PC. Granted, a ps4/xbone will be a lot more pricey, but I don't feel the need to drop cash on one till a price drop hits.
PC would work pretty well if most people had desks in their living rooms, but as it stands I think thats limited to uber-nerds like me who have a desk with two monitors in their entertainment room, and then the TV right in front. Also, my experience is that HDTVs look like shit as PC monitors generally.