For better or worse, systemd has adopted the OpenBSD approach to portability. Nothing is stopping you from creating a systemd-portable project, similar to how OpenSSH-portable makes OpenSSH usable on non-OpenBSD platforms.

As to logind, it may have been a better choice for the long term to do a separate implementation of the public and stable logind DBus API instead of trying to run the systemd-logind implementation without systemd as PID1, but supposedly whoever did the latter thought it was the best short-term choice.