That's nonsense.
I own a 5+ year old iBook G3 (blazing along at a whopping 600 Mhz

) and it has had Jaguar, Panther and Tiger installed. Each of those got progressively more stable and faster. Tiger is amazing on it. There are no noticeable lags during usage. There's no bizarre behavior. It's solid and very responsive. Obviously, I'm not doing any rigorous production work on this thing, but for a casual-use laptop, my iBook is hard to beat. Likewise, I have installed various versions of OS X on older hardware where I work and have witnessed similar performances bumps each time so I know this is not just a fluke in my case.
Where do you get this idea that Apple is gearing OS X so it pushes older systems into the unsupported category? From what I can tell, they're doing a fantastic job retaining compatibility with older hardware--certainly better than what I've seen with Windows XP.