Areas of responsibility are implemented by standalone mechanisms that compose to create the larger integrated system and can:
1) Run in isolation from their upstream dependents.
2) Be replaced individually.
And seriously, systemd isn't one single daemon either. It's a collection of daemons running that communicate over a common protocol. It isn't all-or-nothing, either. Only a few of the daemons are considered "core" and the rest are optional. And, because they use a common documented protocol, even the core components can be replaced. You don't hear about alternatives because nobody has really written any, but it doesn't in any way prevent you from doing it.
I think the legitimate concerns about systemd are things like losing portability of stuff like Gnome to BSD and systemd's short history, but it isn't one giant monolith executable or anything like people say.
What common protocol? Mac OS X doesn't have dbus. It recently acquired XPC, but XPC is NOT a system bus -- it's simply an IPC library. It's neither required, nor universally used, nor is it terminally glued to the remainder of the OS.
> You don't, for example, just replace launchd with some other init system.
You don't? Says who? All launchd does is serve as an inetd/cron-esque daemon. The system daemons themselves continue to vend standard sockets and mach ports, and launchd itself does not pervade their externally vended interface.
> You don't just say "hey, screw Aqua I want to use something else."
Actually, you can. The display stack is driven by IOKit; XDarwin can run directly atop said stack.
Likewise, none of the "Mac-like" features that you want -- for example, automatic network configuration -- rely on any sort of centralized entity that pervades all daemons. There's a seperate system -- SystemConfiguration -- that provides that functionality, abstracted through a set of well-defined APIs and modular and distinct from the remainder of the system.
> * It's a collection of daemons running that communicate over a common protocol.*
Hence it's really rather monolithic, because to play in the systemd universe, everything has to conform to systemd's tightly coupled design.
Yes and no. It's true that OSX uses a variety of separate daemons, that run in different pids.
However they are all "required", "packaged together", "part of one source tree" and "shipped by a single vendor". In this respect they are a lot like systemd.
In fact it is arguably a lot more integrated. Both Python and Ruby are OSX dependencies. systemd isn't even close to that much of a power-grab.
