"There are dozens of things you can do on a Desktop that you simply can't do on a Tablet. I think the fallacy lies in the fact that that Desktops aren't part of the market that study was graphing. "
You have to consider who is using a computer now. The answer is "everybody."
But what are they using them for?
Most people aren't burning DVD's or writing software or novels or even letters. Most people are using them for consumption of media, written or audio/video 95% of the time.
The dozens of things you can do on a desktop don't matter at all to 100 million aging baby boomers who just want to see what the stock market is doing, or what the weather is like "back home", wherever that is, or want to see pictures of the grandkids or check the sports scores.
Whatever people were doing with books and newspapers and televisions in the way of passive entertainment and consumption, they can do now on devices like the iPad, and the consumers dwarf the producers.
People who need a "truck" will still be using a truck, but most people don't need a truck. Now, there will be all kinds of people who will buy a desktop who don't need it, in the same way there are all kinds of people who drive monster 4 wheel drive trucks around the city like a fashion statement.
But the majority of people who use computers won't want or need a desktop.
Market Data 
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