The GameBoy Color was it's own generation of Game Boy. Like the GBA, it was just backwards compatible with every GB game that came before it, with possible exception to the games with special equipment built in.
In this sense, the GBC did not exist direct alongside the GB, but rather succeeded it, though many don't realize this when they talk about the GBC. Additionally, the GBC had a short but full lifespan of three years, which is short compared to normal console lives, but was mostly because Nintendo took an unsure approach with new handhelds after the Virtual Boy, and created much of the new software with sideways compatibility to the existing Game Boy line, for fear of another hand held flop. The platform itself sold about 50 million itself, according to Wikipedia, but it was entirely it's own entry to the Game Boy franchise.
Now, in Japan, there were three original models of Game Boys, whereas in America, there were two. There was the original Game Boy, the brick. Then the Game Boy Pocket, which was succeeded by the Game Boy Light in Japan.
Anyways, aside from the History lesson, I have to say I agree and disagree with you. Nothing Nintendo releases will have increased functionality without being it's own tier, similar to how the DS and the GBA stood at first. Whenever Nintendo releases an upgrade to a console, it shares all the same functionality as it's brethren, with stand-alone additions, built to correct design flaws that could not be corrected at the time of the first model. What I'm saying is that if a new DS is released, any changes will be superficial, and nothing more. However, if a successor to the DS is released, it could contain DS in the name, but have new features and games that are exclusively playable on it, even though it may remain backwards-compatible with the DS's titles.
Edit: Unclebob beat me to it. :p