The point of an integrated system is that interactions between components enhance the system as a whole. This is obviously incompatible with an easy replacement of components.
This is literally just basic systems engineering. I don't even know what you are arguing here. The more tightly integrated a system is the harder it is to replace individual components.
The more tightly integrated a system is, the more involved the contracts between the components become. I seriously didn't believe that a single person here would disagree with that.
A component that depends on one simple interface is far easier to replace than one which depends on twenty complex ones. This seems like the most basic stuff.
You can have a well-integrated system without having tight coupling between components. Interfaces are a thing, and a high level of integration just means having a good collection of interfaces between well-defined conceptual components with well-defined capabilities. The actual software that implements those components is entirely separate.
I don't know what you are arguing. Replacing a component in a tightly integrated system means that the component has to be compatible to that tightly integrated system, meaning lots of assumptions and replication of functionality.
This means components have to tightly conform to the components they are replacing, which obviously decrease modularity as these components are harder to maintain for available system configurations.
This isn't about whether someone can rewrite a part of systemd, but whether you can freely mix and match, which tight integration works against. Obviously
You can replace most systemd components but in that case you have fewer features and probably more code overall. If you run the whole systemd bundle you get more features.
This is literally just basic systems engineering. I don't even know what you are arguing here. The more tightly integrated a system is the harder it is to replace individual components.