systemd is not a monolith.

It's a collection of losely coupled components and services of which basically every single one can be disabled or replaced by another implementation.

In theory. In practice, systemd is a mess of different components that have subtle dependencies on each other. And while the core of systemd is solid enough, everything around it is not.

It's a collection of tightly-coupled components that are functionally a monolith because large distros tend to rely on the various components rather than allowing modularity.