Unless you're operating at Google/FB/Twitter/etc scale, this is not a problem most of us face IRL. We need to stop pretending we have these problems at 99% of the places we work at. Most of us don't. We don't need 1,000 microservices, Kubernetes, Docker, and a terribly unreliable NoSQL database like MongoDb (there are better, more optimal solutions for those companies than MongoDb (Cassandra being one of them.))

You're technically correct, but this case study was really eye-opening to me: http://blog.sagemath.com/2017/02/09/rethinkdb-vs-postgres.ht... They migrated from a RethinkDB cluster (similar to MongoDB) to a single Postgres server and the result was faster. And a single server is very large today, like 48 cores and 384 GB RAM.

And speaking of Postgres HA management, master promotion, etc. there are several tools like https://github.com/sorintlab/stolon available.