It’s a really good idea since apple is abandoning its x86 software. It’s an opening for FOSS to pull in new GUI frameworks/features. Could really help gain userbase.

They use FreeBSD and Darwin open-source code. Darwin is the core of MacOS with the XNU/MACH Kernel. So they are doing nothing wrong that Apple can sue them over it. The trick is writing the code that does what the Non-Open-Source code of MacOS does to run the Mac Apps.

> The trick is writing the code that does what the Non-Open-Source code of MacOS does to run the Mac Apps

And that is where the magic happens.

GNUstep has been trying to provide a useful open source alternative implementation of OPENSTEP for more than 20 years now I believe. I don’t see that happen to be honest.