I feel like the GB/GBC/GBA are pretty much what the VT102 is to writing grid-cell software. The demo scene keeps cranking. Chiptune is still in love with the hardware - especially with the OG DMG, with a pro sound mod. Writing new software for the GBA feels a lot like targeting a tiny, modern ARM microcontroller. The hardware is well documented and understood by the enthusiasts, and it's a stable target.

Plus there are plenty of emulators focusing on different aspects: one is focusing on perfect emulation; another will try to squeeze out performance; another might offer memory inspection and "cheat codes"; this one makes it available on the web. There's beauty in all of that.