All things equal you're always better off ruthlessly killing complexity. Cook (I know you're not rebutting him) is focused on the problem of unavoidably complex systems. A big server running a single instance of Postgres is great, and you should absolutely use that architecture if you can. But real-world constraints can break the architecture: need it running in multiple locations, with a single global view, with strictly bounded propagation times (for instance: because you're running a routing protocol): it's still just a database problem, but now it's a much more complex one.

And then, naturally, you can get complexity just by building stuff on top of that big database server, and other things that depend on those things.