Anything that wanted to pretend to be one of the various daemons can simply speak the dbus protocol.
The internal DBus API that these components use is not stable and will not be stable for the foreseeable future; according to the systemd developers, parts of systemd cannot be re-implemented independently [1]. That effectively makes systemd a monolithic system--its components are tightly-coupled and cannot be readily swapped out.
Note that a system can be both monolithic and modular--systemd, X.org, and the Linux kernel being examples. Systemd gets a bad rap about it because the components that make up systemd were once loosely-coupled, and the need for the new coupling is unjustified.
Comments
The internal DBus API that these components use is not stable and will not be stable for the foreseeable future; according to the systemd developers, parts of systemd cannot be re-implemented independently [1]. That effectively makes systemd a monolithic system--its components are tightly-coupled and cannot be readily swapped out.
Note that a system can be both monolithic and modular--systemd, X.org, and the Linux kernel being examples. Systemd gets a bad rap about it because the components that make up systemd were once loosely-coupled, and the need for the new coupling is unjustified.
[1] http://www.freedesktop.org/wiki/Software/systemd/InterfacePo...