I've been wondering the same thing. Two ways this can go in my mind:
1) the supernet is built out and will be free and we won't worry about how many bytes we're shovelling across the link. If so, then why not get served tiles by a mapping server? Great advantages like the map data exists in one place and can be updated easily. But, even so, there are still plenty of unconnected applications and lo-speed link applications, and always will be.
2) free, ubiquitious supernet or not there will always be applications that don't or can't use it.
So, my conclusion is that desktop GIS is not dead.