And the GBC was replaced by the GBA due to competition from the Wonderswan. Time is not a good reason, I think, for judging a game system as being a successor or a 'stopgap'.
I call the GBC a stopgap system not for any single reason, but for a lot of little reasons that all add up. The GBC is a system that only lasted a couple years on the market, had hardware basically identical to it's predecessor aside from a specific bump, had a basically identical form factor to it's predecessor, and generally used exactly the same peripherals as it's predecesssor. It didn't even add any new controls, as is typical for a new generation of a Nintendo handheld. There's also the fact that a not insignificant portion of Game Boy Color
games were backwards compatible with older Game Boys. That is highly atypical for a new platform of that time.
Even the Wii, the Nintendo system most similar to it's predecessor, still had much more differentiation than that. Even if the main chips were largely the same, The Wii introduced the "Starlet" CPU core, which has no analog in the GameCube and significantly changed the way a bunch of things worked. It looks very different from a GameCube, and the GameCube slots are clearly there largely for backwards compatibility.
The GBC really has far more in common with the DSi and New 3DS than with the Wii.
I think this conversation may deserve it's own thread. Give me a few minutes, and I'll make one.
EDIT: New Thread:
http://www.neogaf.com/forum/showthread.php?t=1117295