The creator of systemd specifically made the choice created an architecture leveraging the best parts of the Linux kernel. So the way the program works, depends on the Linux kernel.
It means that systemd should be a fairly small program when compared if it needed the right abstractions and the architecture would probably have been different too.
Wayland is probably the same, I'm sure the code base is very small in comparison to X. For example keyboard handling is mostly handled by the kernel I believe.
I was already wondering how long the Debian project could keep the kFreeBSD port alive. For now in Debian they will probably let the user choose to use systemd or some other init. Maybe systemd will eventually the default with the Linux kernel.
kFreeBSD had ZFS and PF, what other user visible ? Will btrfs fix the biggest feature ?
It used to be true that academia developed new TCP/IP extension on the FreeBSD kernel first, this isn't true anymore. This is now Linux too.