When systems people talk about system design, they talk about the whole value-generating system and not just software or hardware. This typically involves people and brings in issues like the Conway’s Law. A tightly coupled team with little independence in terms of workstreams will produce a monolith regardless of what the architect dreams of, for example. If you have two sets of users with diverging regulatory and organizational needs (people who maintain the registry of citizens vs people who issue identity documents), you will have two separate data stores regardless of how much sense it makes to have just one.