It's not like PostgreSQL hasn't been in development for close to 30 years, covering basically every use case imaginable just through millions of deployments.

In addition, SQL in itself is a proven technology. The reality is that most problems you might think about solving with specialized databases (Big Data TM etc) could probably easily be solved with your run-of-the-mill RDBMS anyway, if more than five minutes are spent on designing the schema. It's extremely versatile, despite just being one layer above key-value storage.