In fact, I would go so far as to say that the only time when a massive change to the desktop environment is warranted is when there's a massive change to the physical method that people use to interact with computers. The widespread availability of the mouse gave rise to the now-familiar WIMP paradigm. The introduction of multi-touch triggered another round of innovation, but I doubt that this can be backported to keyboard-and-mouse devices any more than Photoshop can be backported to mouseless terminals.