Nintendo's new 3DS-focused Direct was today

[A Black Falcon]

Yeah, you're probably right -- if the NX really takes off, 3DS support will probably end sooner than the current plan. On the other hand if it doesn't, 3DS support will be extended. Both fit right in with past Nintendo actions. That would make sense.

Heck, even when the Color was going strong, there were still non-color GB games coming out in droves.

This isn't really true, though. There were indeed a lot of games that supported both color and B&W, but there are VERY few B&W-only releases from after the GBC's launch; developers almost immediately switched over to dual-mode games. I presume you're thinking of all those dual-mode games, but since they also support the new system that's a slightly different thing I think. And after the GBA's release, Nintendo pretty much completely stopped releasing first-party GBC games almost immediately. The only first-party GBC games released after the GBA's launch are Western launches of a couple of games that released in Japan earlier, something that disappointed me at the time for sure. On the other hand, the GBA had major releases for several years after the DS's launch. I liked that a lot more than the short GBC lifecycle...