You're the one lacking imagination. OS X could be packaged in a lighter form while maintaining functionality.
The problem is that all the OS X applications wouldn't be immediately upgraded, and wouldn't be very usable as-is, out of the box, which would leave a frustrated user base, and bifurcated application world. Some applications would work better with touch, others with a mouse... and you'd lose the whole portability and simplicity that's the beauty of the iPad. And that's what people primarily interface with - applications, not the OS.
It's not the best analogy, but... Lots of people had invented cars before Henry Ford brought out the Model-T, but that car was certainly revolutionary. There were more capable cars on the road, but none that had the combination of usability & affordability that brought it to the masses.
All that said, I think there could be a future iMac that optionally folds down to a configuration like a drafting table... but that's a completely different product, and won't be selling for $499+.