It matters if they're separate programs or not, and they are. You can pick and choose, and distros do. Systemd is not a monolith, it just isn't.
How many pieces of software is nginx or Apache, where every feature is an optional plugin?
E.g. explorer.exe is not replaceable on windows so the display server and that is one software. (The kernel is NOT though, because it can surely run in a headless way). It's not that hard of a concept..
You can absolutely run systemd-init without systemd-journald, and some distros do.