Honestly, there's never been a better time to be writing games for those platforms. The SDK's are much improved from the proprietary stuff available at the time and the hardware is very well understood. People have been pulling off some utter witchcraft on the N64.
Kaze and James Lambert are amazing. Kaze's new Mario game looks better than a first party Nintendo title, and James's engines pushes the console to its absolute limits.
I've been thinking about giving it a go myself. It's such a fun and nostalgic console, and the limitations are fun constraints.
The code archeology is really cool too. Seeing Rare's Dinosaur Planet boot up and play after being a lost title. Decompiling all the original titles. Building sequels to Ocarina of Time and Majora's Mask. It's such a fun scene.
Then there's Analogue 3D and ModRetro too, which make it fun to play as physical hardware.
Best of all: there's no hardware rat race.
The N64 is never going to get any faster, and emulators can run on just about any potato these days. People have realistic expectations of what the platform can do, so there's no need to spend insane effort making the graphics look better. It's always going to look kinda crappy, and that is Just Fine.
This lets game developers focus on the actual gameplay itself. No "Generic Shooter vol. 26 - now with slightly prettier water!", but innovative stuff focusing on the narrative and on novel gameplay elements.