[edit] I think I wasn't clear. Scaling the direct on-screen manipulation up to laptop/desktop fundamentally changes the physical motions required.

The Magic Trackpad is indirect scaled motion like the inbuilt trackpad on a laptop. It's size is still within the range of easy and quick wrist motion unlike touching the desktop screen itself.

There's even push back against ever increasing phone screen sizes. They've gone beyond a finger's range of motion.