I think you're overcomplicating things a bit. They all play music and that's still what most people buy iPods for. You still have basically three categories of device: small size, mid-sized, large. The amount of storage space included mostly matches the physical size of the device. The extra features on different models isn't really that important. Seems like they just need to give owners of older models some reason to upgrade and it doesn't really matter that much what those features are.