Tablets are small, cheap, portable, silent, generally have a camera, speakers, and microphone which gives them built in telephony features, and an easy UI. I can buy a decent tablet for less than $100 on eBay or the local discount store.
What are the redeeming qualities of a PC? Tactile IO: there is something to be said about a typing with a keyboard as opposed to a flat screen. Better audio and video quality: its just has a bigger screen and a better presentation system then a system with micro speaker and 7" screen. Deeper interfaces. For all the issues that Window's has its can do more then a tablet OS. Modularity, Upgrade-ability, and repair-ability: If I want a bigger screen I buy one for my PC. If want bigger screen in tablet I have to buy a whole new machine.
For these issue PC's will always be around. But they will get pushed in the development and special needs category in the next decade or so. Tablets will keep coming down in prices. Operation system like Ubuntu that give tablets more of the PC's feature. One off devices like the Roku and game systems like the XBox One will take up the home media center and entertainment.