My Grug-brained hill to die on is that there are many pieces of technology like the LM741 opamp, the Rover V8, Morris A-series and Peugeot XUD engines, and a big SQL database running on a thumping great Unix box, that have remained essentially unchanged for decades, worked well when they were new, and are still a perfectly good solution today for so much of what you want to do.
You know what they have in common? A lack of complexity. Lovely simple technology, fix it with a hammer. Drive it to the moon and back. Drive it across the desert. Bring tools though!
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.
But then if you keep the components simple - Postgres is a well-trodden path, for example, then you still manage to lop off a lot of complexity. And then of course you can keep the composition simple too - maybe you just need three thumping great Debian boxes running Postgres in three locations, maybe you just need some IPSec tunnels (complex to set up but again a well-trodden path) between them instead of what the latest SDN hawtness is, and so on.
I find it amusing that you classify a SQL database running on a Unix box as simple and unchanged in decades. Databases are some of the most complex pieces of software we regularly use and subject to highly active research. The operating systems running them and the hardware running those of course too.
From the point of view of you the database user, you the guy that sets up the database, that creates the SQL, that does "sudo apt install postgres", it's simple.
Could you repair the little Briggs and Stratton in your lawnmower? Almost certainly. You can easily hold a mental model of how it all works in your mind, and diagnose and repair any fault.
Could you actually design such an engine? Where would you start?
You know what they have in common? A lack of complexity. Lovely simple technology, fix it with a hammer. Drive it to the moon and back. Drive it across the desert. Bring tools though!