It runs its own logging system with non-standard interfaces and formats. It runs its own DNS resolver with non-standard behaviour. It maintains compatibility only with a narrow range of udev versions, which in turn maintain compatibility only with a narrow range of kernel versions. And all the d-bus interfaces between these pieces may change at any point without notice. So you can't replace any piece of it, because even if you provide your own component that implements one of the systemd d-bus interfaces, you've got no forward compatibility.