Yeah oddly enough that seems like the diciest part of the project to me. The Cocoa API is vast, reimplementing it isn't necessarily rocket science but would require a pretty huge amount of labor.
And you're right - the Cocoa API continues to evolve, so if the goal is "source compatible with latest and greatest macOS APIs" it's hard to see how a small group of open source devs can out-run the sum totality of every API engineer at Apple.
I have similar feelings about cross-platform tools like Flutter as well - the idea in a vacuum is reasonably sound, but then you run into the basic scale problems of maintaining compatibility with a moving target - a moving target that has >10x more staffing than you do.
And you're right - the Cocoa API continues to evolve, so if the goal is "source compatible with latest and greatest macOS APIs" it's hard to see how a small group of open source devs can out-run the sum totality of every API engineer at Apple.
I have similar feelings about cross-platform tools like Flutter as well - the idea in a vacuum is reasonably sound, but then you run into the basic scale problems of maintaining compatibility with a moving target - a moving target that has >10x more staffing than you do.