I saw someone asking this at one point somewhere else, and I was thinking about it, and it really is interesting to think about. Would you consider the Game Boy Color a full on successor to the original Game Boy Color? Or more of an upgrade?
I think it was originally released to be a semi-successor like the more recent DSi or New 3DS, but in the end became more of its own separate platform with lots of exclusive games being released to it. I really think they were probably focused on Game Boy Advance at the time and knew it'd still be a few years off, Game Boy sales were probably slowing down so they wanted something to release in between, and it just became so popular that they ended up releasing a ton of exclusive games for it as if it were a new console.
I think it was originally released to be a semi-successor like the more recent DSi or New 3DS, but in the end became more of its own separate platform with lots of exclusive games being released to it. I really think they were probably focused on Game Boy Advance at the time and knew it'd still be a few years off, Game Boy sales were probably slowing down so they wanted something to release in between, and it just became so popular that they ended up releasing a ton of exclusive games for it as if it were a new console.