As long as there exists work that requires sitting down at a computer and investing oneself in highly complicated tasks, the desktop will exist. If technology has evolved so much (e.g. metro took over) that those complicated tasks can be broken up across multiple non-PC [metro-ized] devices, then why would it matter if the desktop still existed then?
Give me a Metro development environment, then -- the desktop is Development Central.
If you argue a Metro device can remote to a VM running VS, then I'd say that VM is desktop... no where to go.