The two lines you mention do cut off at some points, though. The DS no longer had backwards compatibility with the GB and GBC, the DSi with the GBA, although the 3DS re-added GB and GBC, just digitally. The Wii U also doesn't support GCN discs. All of these, however, work pretty well with homebrew like mGBA and Nintendont, they're just not available officially.
Yeah, I agree with that. I was just trying to point out how Nintendo's BC has never been as complete as it could be, for whatever reason it may be, but yeah, they've been pretty good at guaranteeing last-gen compatibility so far. Let's see how it goes with whatever will succeed the Switch.