Upstart and OpenRC both came years prior. As a matter of course anything which was self contained and standardized could have allowed interoperability. Systemd

Upstart and OpenRC both came years prior. As a matter of course anything which was self contained and standardized could have allowed interoperability. Systemd just actually convinced everyone to standardize not least because its integration with other parts of systemd provided both a carrot: standardization and simplicity for distro maintainers and a stick: the extra work required to use only part of it or especially if one used none of it and was expected to both write ones one unit files AND integration with stuff like gnome which was and remains the most popular singular GUI.