[gh02t]

What you're describing is all quite similar to how systemd operates. Your definition of "monolithic" seems to be that the components communicate with each other. My definition is that the components are difficult to replace. Systemd makes it easier to replace components, because the requisite interfaces are clearly defined.

MacOS is quite tightly integrated, it's what Apple does. Darwin is a bit different, as it's much more BSD-ish; with that I agree. But MacOS is a lot of stuff on top of Darwin and replacing any part of that would require tons of very MacOS specific code to do. Apple most definitely does not intend for their users to be able to replace logind or Aqua or pretty much any core service.

Probably a better way to phrase it would be to consided how much effort it'd take to run Aqua on BSD. You'd have to port basically everything that is MacOS. To me, that is what a monolithic system means.