This is because largely speaking, the Wii is the same hardware than the GameCube (but with improved speed). That's why the Dophin emulator, originally a GC emu, was able to extend easily to also cover the Wii.
Right. That's why I didn't skip that generation in my example. My hunch was that as a general rule for those earlier generations that it's not possible to emulate the immediately previous generation unless there was special support, but maybe you could go back 2 generations. This project being an exception. That's what lead to the
Wii -> GameCube (similar hardware) -> [not N64] SNES -> NES (demonstrated by this project)
example. Heck, if console manufacturers kept two separate branches "every other generation" emulation (odd/even generations) and then made sure each console generation had extra hardware to play the immediately prior generation's games (as some sometimes do), then every generation could play every other generation. That would be nice for players, but maybe not nice for the companies though.
Arguably the Wii is emulating parts of the GameCube. I was pretty sure that certain MMIO banks on the GameCube were being emulated by starlet rather than having hardware blocks like the GameCube probably did. That's how custom MIOS hacks floating around the internet allow for USB controllers and mass storage from GameCube games.
No need for this, it's natively supported and runs game cube games (and with the right setup, GC homebrew)