It is rather like a car manufacturer just putting in a new Engine, updating the seats and dashboard and doing a relaunch. They always change the externals because that is what the customers expect.
That's actually a really good analogy you make there! Apple could have easily changed things at the last minute, especially with the new breed of Android smartphones already making the 4S outdated, but they didn't because they knew that most Apple sheeple would buy it because it's an Apple product.
To be honest, I'm actually disappointed they didn't announce a new iPod Touch and if they do announce one in the near future with increased storage especially with the top-of-the-range version, I will be pissed as that's the only Apple product I would ever buy!