Another thing to keep in mind is how hard is it to get Widows XP users to upgrade? And much harder will it be to get Windows 7 users to upgrade? Most people will be contempt will 7 for a very long time. Microsoft needs to do something radical here - like moving into a new ecosystem. Windows 7 will be great on desktops for a very long time, probably longer than XP will be. I think MS is betting a lot of their sales on tablets and ultra-books with touchscreens. Anyone with a desktop who upgrades will be icing on the cake. I also think MS will move to a shorter release schedule, especially because of the yearly releases in the mobile space, and the with the shared kernel of Windows 8 and Windows Phone I could see them releasing yearly updates for both.
The average user no longer wants a desktop. They want portable and ease of use - something windows 8 will be great at. Consumers can buy a tablet for use throughout the day (I can see students using it to take notes), and then when they get home they plug in a mouse and keyboard (maybe a external monitor as well) and now they have a desktop to do schoolwork. For the average consumer, they will have a tablet, laptop (with an attachable keyboard like the surface), and a desktop all in one device! You don't get any easier than that. Always having all the music, pictures and documents with them. That is where the consumer market (and Windows) is headed - converging into one device.
For us power users, we need our desktop. We need computing power for everything we do (along with at least 2 or 3 monitors
). For us, the desktop is not going away. If you don't like the new start screen (really the only negative) and you don't foresee getting use to it, then use 7. It'll last you a long time. Some people actually like the new interface on desktops, not necessarily all of Metro, but I do like the start screen. I will probably never use any Metro apps on my desktop, but all of the other improvements make it a worthwhile upgrade for me. I still have my same desktop from 7 and I never used the start menu anyways because I have everything pinned to my taskbar. I think most of the kinks in Metro will be ironed by Windows 9, and will be released much faster than 7->8. Windows will always be coded on the new Windows in development (MS dogfoods all their own software), so we are not going to see any radical changes to the desktop anytime soon.
tl;dr - People don't want to upgrade (not off XP, even more not off 7), so MS is moving to a new market, one which is converging consumer devices. For the average consumer, desktops are on their way out (but Windows will still always have the desktop for power users).