alt Hacker News> 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 well as optimistic eventual consistency.
There's just too much scope to be the best in class at everything, and even if somehow you did it, the maintenance cost of all the options would be huge.
Best you'll get is as computing continues to increase, the cost of using less than the best becomes more reasonable. There's a lot of database applications where any reasonable database works, and there's a lot of databases that are reasonable in wide application. That may well lead to fewer databases being available, but it's unlikely to converge down to a single database. Just as most engineering domains don't converge down to a single solution for all applications.