I agree with most of your comments - certainly, more processing more would be required for the full blown.
As for the future of the GUI based on cartoons, morphiong rather than photo-realism - you may well be correct. But 3D models would not stop designers from implementing any form of 'theme' for the desktop. It would merely make it far more impressive to the end user.
Having a proper 3D desktop would allow for a fully interactive experience. It would be akin to the change from a 2d games to 3D games back in the 90's.