I understand, and even appreciate, that they don't let too much customization on their default programs. But why can't you run a different program? It makes no sense.
Depends on what you mean by "run a different program". You can't really replace WindowServer on macOS, as it is pretty much macOS itself. macOS is not really as modular as Linux or even Windows. There are things that hook into it to give extra behavior (Helium, Afloat, Magnet, Amethyst, chunkwm, several others in the AppStore), but replacing it is not really practical, I believe.
> You can't really replace WindowServer on macOS, as it is pretty much macOS itself.
I find this very foreign. As if a unix system didn't let you change the shell!
This is sad to hear, because I'm a happy user of a headless macOS (not set up by me), to which I ssh almost daily for testing purposes, and it is a fairly decent unix system. It has a few idiosyncrasies, sure, but those help to make your pipelines more portable. I always supposed that the GUI would be just a regular program running on top of that unix.
The window manager is definitely a "default program" that is probably tightly integrated in various points of the OS. AFAIK, you can't even use the OS without having Finder running (not the actual window "Finder" but the process) as it's also involved in lots of things, like the desktop and such.
I believe Finder is not technically necessary, as seen on Apple's installers and restore mode (which are a smaller version of macOS). You can kill it with Force Quit, for example. It is indeed also the provider of the Desktop, but everything else works: Dock, Menubar, Command+Tab.
It is launched via /System/Library/LaunchAgents/com.apple.Finder.plist
> Can't you just use a different X?
Lol, this is Apple we're talking about, the company famous for not allowing people to customize things to their own liking (for better or worse).