I think that's where it transitions from a solid system to a subpar system and it's exactly that part that's relevant when we think of distributed systems.

Postgres is solid and a good choice because most times you don't need more than one database and not dealing with complex enough problems that would require a distributed architecture. In reality, you always do, it's just that you're fine with a non-bullet proof solution. For example any real app will have distributed state that spans across UI and database (think of as simple as React's UI state to more comprehensive state management like MobX and finally a database.

So the distributed aspect of systems is actually pervasive and goes beyond what you'd consider a database. And when we talk about the future of distributed system, it's this aspect that is the most relevant one to start with.