Since 99% of the current windows apps are desktop applications, I don't see it disappearing anytime soon. Metro Apps are very limited in what they can do and many desktop apps cannot be moved to Metro without severe loss of functionality.

I seriously doubt that this will happen anytime soon. One reason why Windows has grown so big is compatibility with its predecessors. That will not change. I do not see any difference in desktop behaviour at all when I run them in Windows 8. Often I even forget that I run Windows 8.

The Metro apps are intended to be cross (hardware) platform apps, so they can migrate to phones, tablets, tv's, xbox and whatever they may think of. As you can see in the store, almost all apps are dedicated single purpose tools. The new upcoming version of Office is a desktop app which intergrates with the cloud. It runs on Windows 7 as well - - even with the new UI.