The reason we buy newer models is because they sound better
Newer models are usually technically superior, and have more waverom (thus more sounds & better sound quality), better styles (because of the newer & improved sounds, and more polyphony allows programmers to create more complex styles as well), I think that most keyboardists (including myself) seek the ultimate in sound realism, we want our arrangers to sound as close to the real thing as possible, and every new generation is a step closer to "The real thing".
And newer models will have improved & new features to even make playing keyboards more fun and easier and more efficient then before.
Of course it still depends on the player how good the music sounds, but the sound itself and the quality and realism of it will depend on the keyboard you use, and a great sounding keyboard will make you sound a lot better, but wont make you play better.
I'm already looking forward to what the future has in store for us "electronic" musicians, and what improvements will be made on future models.

Greetz ,
Marcel