This is the reason why I don't like systemd. Make no mistake, systemd has some very good ideas. But when they say that it's modular, replaceable, maintainable etc, this is what actually happens. It's all that in theory. But the reality is very different. GNU Shepherd (guix's init) for instance, is something you can hack deeply using Guile Scheme - sort of like Emacs with Elisp. Much of the Guix OS is like that. Strongly integrated with Guile Scheme glue. But flexible and modular enough to be swapped out easily. Imagine doing the same with systemd init.