Tilt,
I think more of it being like....

D&D+AD&D is a Model T
2E was the mass produced sedan of the 50s
3E was the all-terrain vehicle
4E is the "personal 4WD people mover never intended to get off the suburban roads"

Yes they are all cars... but they are very different beasts. So yes, they are all RPGs and are all based on those first ideas put out by G.G. But they are not the same. Not even close.