As much as I agree with your sentiment, it still remains one of the most popular databases out there. All us techies can learn something from the hype they managed to create.
Here's a couple disproving your claim:
https://www.scalebase.com/the-state-of-the-open-source-datab...
It seems ridiculous, really -- developers commit to something because it saves them hours at the start, versus considering the months of work following -- but many technologies followed this curve. MySQL was a lot easier to start with than pgsql, so it won the early battle. PHP was a no-brainer to start with, etc.
The lesson is that your tool should default to essentially no configuration, no security, and simple beginnings, and it tends to pay off.
Though I really like RethinkDB, the lack of automated failover handling is about the only concern left keeping me from them. Because of the tooling around it, I'm using ElasticSearch om my next project over both of them. I've also used Cassandra in the past, which was great in some cases, harder in others.
For the past couple years, I've tended to pair an SQL database with a key-value/document store depending on the needs. SQL is the canonical source, and saves are mirrored to the distributed database for wider reads. Most no-sql databases sacrifice parts of CAP/ACID in different ways, and can fit certain growth needs.
When you are faced with paying a over $30-50k/month (hardware, licenses, bandwidth, redundancy) just to host your sql database on hardware big enough to handle the load, or distribute... distribution becomes an option. For a company with under a million a year in gross revenue, spending $600/year for the database alone is not an option. Even for larger companies it's probably not the best idea.
There's something to be said for ease of configuration and administration in that mix.
That's a lot. Could you elaborate what kind of hardware and software one gets at that price level?