ever hear of video games
PC games still beat all the consoles boxes pants off. Battlefield 3 sold 10's of millions of copies already, (a large portion on PC) and no you can"t play it on an iphone. Anyone who enjoys gaming with HD quality video (there are 10's of millions of us stil) is going to keep upgrading their PC, at least every view years. There are also tons of other uses that you are forgetting. I can use my PC to store and view TB of videos from any other PC or screen in the house. Home Security cameras can be hooked up to a PC and you can store months of surveillance footage, and even check in on your house/dog/kids remotely from work, you can create and mix music, and even write articles, stories etc (with full keyboard) all stuff you can't really do on a cheap tiny screened mobile device.
Don't believe the same BS story that detractors have been pushing for years.... every F*&king year I hear it a dozen times..."blah blah PC's are dead blah" but amazingly every single year since they started being sold MORE PC's are sold every year than the last one, even despite economic downturns. Sure on bad years the market only rises by say a fraction of a percent instead of several, and some counties with really bad economies might see a dip in purchases, but worldwide, PC makers are always making more PC's with and there is NO EVIDENCE to any end of this in sight. Sure as Laptop prices drop, and their screens get better you may see some PC's switch over. Tablets and phones will not replace PC's. And more likely than that, I would wager that the trend will be that people over the next decade will just start to have a PC AND a laptop/tablet. PC's will die when video games and Porn die, and both of them (like PC's just seem to expand EVERY year.