[thayne]

> and (especially a few decades ago) can offer cross-platform/backwards syntax compatibility using a shared code base.

This is still very much an issue. For the shutdown and reboot case, the main reason those symlinks is exist is for backwards compatibility for existing programs and scripts (and muscle memory) that assume there is a shutdown or reboot command, and compatibility with systems that don't use systemd.

Another way to do that could be to use a shell script that execs systemctl, but that requires a separate intermediate shell process, which may have its own compatibility issues.

Another use of argv[0] that isn't discussed at all is putting a hyphen at the beginning of argv[0] for login shells. For example if bash is invoked as the login shell argv[0] is "-bash". That probably wasn't a great design decision, but changing it now would probably cause a lot of breakage.