> that nobody is actually required to use at any given time
But that's the very problem with systemd! As time goes on you're required, whether by systemd itself or by the ecosystem around it, to use more and more of it, until it's doing not only service management but also timezones, RTC, DNS resolution, providing getpwent/getgrent, inetd, VMs and containers, bootloader, udev (without adding literally any benefit over the existing implementations), ... oh and you also have to add significant complexity in other things (like the kernel!) to use it, like namespaces (which have been a frequent source of vulnerabilities)...
But that's the very problem with systemd! As time goes on you're required, whether by systemd itself or by the ecosystem around it, to use more and more of it, until it's doing not only service management but also timezones, RTC, DNS resolution, providing getpwent/getgrent, inetd, VMs and containers, bootloader, udev (without adding literally any benefit over the existing implementations), ... oh and you also have to add significant complexity in other things (like the kernel!) to use it, like namespaces (which have been a frequent source of vulnerabilities)...