Systemd is literally not a monolith because it's not one big piece of software. It's a collection of software. For example, systemd-boot has basically nothing to do with systemd-init.
Yes, some software is more tightly coupled than others, naturally. systemd-init and systemd-journald work tightly together because logging is a core requirement of any init system.
Yes, some software is more tightly coupled than others, naturally. systemd-init and systemd-journald work tightly together because logging is a core requirement of any init system.