From the published sales figures and stock prices of every major PC manufacturer with the exception of Apple. 
This is oddly more akin to what others have said, there is a decrease in need for people to upgrade. Only people who are still upgrading their desktop systems more than every 5 years or so would be gamers, and even then you have some people that only play World of Warcraft, and it's not like that's a very demanding game in current computer terms.
Apple's stock prices aren't going up due to their cylinder computers, that's for sure...
Oddly when I was hired on at my current job, they supplied me with a Dell tower running Windows 7 with a xeon in it (so hardly a typical desktop computer) and a macbook pro (which I hate). Granted I setup up a dual boot of Debian on that Dell tower and use vmware on the mac to use Debian there, but the point is, companies still buy desktop systems for their employees, even when supplying them with laptops as well. So it's not dying by any means, people just don't need to upgrade that often anymore, or if they are, they upgrade the desktop they have instead of buying a new machine.
