Unless you give me a more concrete example, I don't really think this is true.
For example, right now neither my desktop nor my notebook uses `systemd-networkd` and it works fine. Actually, since I use NixOS even basic things like `hostnamectl` or `timedatectl` are "disabled" (they work in read-only mode since those configs are managed by NixOS). But I do use `systemd-bootd` (so I can get boot information from the early UEFI until user space, try doing the same thing with GRUB+sysVinit).
The only two components that I know it can't be disabled in systemd is the init part and journalctl (because all the other parts of systemd depends on it).