That's fine, but the frameworks are not installed with macOS. In order to get them, one must sign up for a free developer account, at least, and install Xcode, which is massive, but I'm not sure how you can say it is part of the OS when most people never install it.
The system frameworks e.g. Cocoa, Foundation, CoreServices come with every installation as binaries. These are the building blocks of the GUI and mac default apps. They act as the abstractions to the underlying BSD and Mach system and basically what define a macOS system. [1][2]
My point is I put more value on the closed source parts more than the core OS and even the GUI itself. If these were to be open sourced, practically means possibility of source compatibility (even binary compatibility) on Linux and that is huge deal for the public. On the other hand, I'm not sure how can anyone besides compiler and driver developers can benefits from Darwin being open source.
