Earlier quoted context omitted.
> What queries does it accept? How does it persist data? How does it manage replication and partitioning across multiple servers? Are questions with many answers and the right one varies by application. nothing prevents to build single database which would cover all such answers. Its engineering, funding and distribution problems: no-one built it yet.
Practicality and wisdom prevent trying to fill all the database niches with a single product. Even if you did it, I'd be surprised if a single code base is optimal across the spectrum from resource constrained microcontrollers all the way up to IBM Z Series mainframes and everything in between; along with the full spectrum of persistence from in-memory MRU (or similar) caching to full two phase global consensus as we…
is a marginal niche, but everything else can be handled by one DB. You think we have PG with extensions (including duckdb) already covers most of the ground, making something like that distributed is also feasible task (there are projects), architecting system to make core engine embeddable is also feasible task.