You’re complaining about something different. If there’s a bug in systemd itself that makes your system unbootable then not much you can do except not upgrade. What the parent is talking about is the fact that services are organized into base service files and overrides. Base files live in /usr and are to be modified only by the package that owns them. Override files or full-replacements live in /etc which is for the local administrator. Service updates are now always a safe operation.