I'll say this. To be a succesor, for me. The system has to have higher than single digit original exclusive games. And most have to be close to million sellers. For me to count it as a new system. Because than, people are looking at it, as a new system. Rather than some upgrade, like the GB Pocket/light was.

Only I'm not looking at it from anyone's perspective other than what Nintendo's own information shows and that is GBC was and is an upgrade, good on them it performed like a successor still doesn't change what it was and that is a redesigned GB.

I always viewed the GBC as an uprade / compliment to the original GB lineup, as while it was technically new hardware and GBC-designated games could only be played on it, it served as more of a bridge to the GBA which was the true successor.  Plus the GBC was only around for a couple of years before the GBA was released, and not long after that the GBC was pretty much done as very few games were released for it and sales dwindled in the wake of the GBA release.

GBC is the upgrade just like DSi and 'New' 3DS rather then a true successor.

Actually, yeah! N3DS is a successor, its just a bad one that's failing to justify its existence. It was a wasted opportunity by Nintendo, a New 3DS should be more powerful, have dual joysticks, and higher resolution screen.

GBC made sense. The move to color alone was a game changer, on top of that it gots a specs boost and greater storage capabilities for more advanced games.

If you have to buy a completely new device (not just an add-on) to play new games, its a new platform. Imagine if someone bought you a GBC game but you own a regular GB. You don't need an upgrade, you don't need the GB Pocket (a a redesign), you need a new console called Gameboy Color.

By the way, I don't expect to change your mind. I'm just pointing out you're ignoring a lot of obvious things that make it a successor.

That's semantics in my opinoin.

GBC was just a more desired upgrade, the color alone made a huge difference and it a was cheap new device.

N3DS is basically just a performance boost and a nub. It doesn't appeal to the average consumer, doesn't feel necessary.

I'd say successor, only because of the fact the vast majority (75% or so) of its games were exclusive to GBC.

Backwards compatibility with the GB is irrelevant to whether it's a successor or not, not sure why some people are using that to justify it was an upgrade.
The PS2 was fully backwards compatible with the PS1, but no one would argue that the PS2 was merely an upgrade to the PS1 and therefore we should combine their sales.

To me it comes down to forward compatibility, can the old system play the new games? If the vast majority of games can't then it's a successor, if the vast majority can then it's an upgrade. It's not enough that some games can run on both systems, that just implies that the game as been programmed to run on both systems.

In the GBC's case,  all forward compatible games (GBC games that can run on the GB) were released after the GBC was released, implying that forwards compatibility had to be programmed at the game level rather than it existing at the system level. Therefore it was a successor,  with some games shipping with a backported version packaged together on the GBC carts. 

I always have to laugh when people want to categorize things, even at professional level. No matter what you want to categorize, you always run into things which can not clearly be defined, different people have different opinions. Then some people (the government for example) work out a rule but the rule will always face opposition and always is exposed to change.

It's a cancer of modern society. We want to categorize everything but things can't really be categorized. You can only have your own opinion but even then, you end up arguing with yourself. Remembers me of categorizing races. At which percentage someone can be categorized as not of mixed-race. Is someone with 75% of mongolide race to be categorized as mongolide? Or does it need at least 87.5%?

In practice, it was a successor with full BC and a limited dedicated library. GBC games could only be played on a GBC at the time, and wouldn't run on the normal Gameboy systems. That said, some games were coded to support color (ie Pokémon Yellow).

I'd also like to note that this wasn't one or two titles that were exclusive, like the New 3DS. This was 160 licensed titles that were exclusive to the Gameboy Color. A number like that is enough to call it a successor, whereas the status of the New 3DS is arguable.

History lesson: The GBC had two classifications for its games. Class A games would support color when played on a GBC, but would still run on a regular or Pocket Gameboy. Class B, on the other hand, was entirely exclusive to the GBC. Class B games were easily distinguishable, though, due to their cartridge design (transparent with a convex Nintendo logo as opposed to solid colors with a concave logo).