Once upon a time there was a market for a household electric motor. You could connect it to various devices, like fans and washing machines, as needed. The cost of the electric motor was so high and they were so bulky that you couldn't afford to put one in every device that might need it.
Digital computers have followed a similar path. They used to fill buildings and service the needs of a government or large corporation. Once they fit under a desk, we would buy one for a whole house and use it to replace all kinds of purpose built machines. Now that they are so cheap and tiny, we can afford to pair them with form factors better suited to specific tasks.
They won't die, since some folks will always need a keyboard and screen mated to local storage, but they'll never be the single solution to almost every computer problem again.
I work on a PC all day long a lot of times. Kinda hard to run test programs over a serial or wired ethernet connection to telephony equipment with my iPad. Likewise working on Excel or Word documents aren't really easy to work with on my iPad either.
They aren't dead they just don't need to be upgraded as often. Windows 7 is going to be another XP as it'll be usable for another 5 years easy and this is only in the consumer market. In the business side of things windows 8 is RUBBISH and Mac is only slowing being adopted and mostly in the broadcast media industry.
First off I could care less what the rest of the world does and likes. I love my 27inch iMac! the screen is awesome and it rocks! There is no way in hell I could do what I need to do on my Xoom tablet or an iPad for that matter.
I use my Xoom to surf the web, watch Netflix and so on.
What the rest of the world likes only becomes an issue when the demand for PCs drops enough that the prices go up and only specialized customers can afford them. PC users are currently benefiting from 30 years of mass consumer spending.
Many people will want real estate on their monitor and a full-sized keyboard. I don't care if the monitor is a projected hologram and the keyboard is projected onto a flat surface in front of you with lasers that track your fingers' movement (which a device the size of deck of cards could do)--you're going to have to have that for many, many applications.
Bought my son a mid level laptop (with Windows 7 Professional) for college. had to search the small business section since a search for W7 didn't come up on the website. He didn't like the tablet/i pad style and wants to do low low level gaming on the go as well as loads of school work. The Dell will get him through the next four years and I'll upgrade with Windows 11 on his hologram all-in-one device in 2017.