>I would argue that since both Mysql and PostgreSQL are JSON document stores with mostly the same capabilities when it comes to querying and aggregation...

I agree with your overall argument of PostgreSQL & MySQL >> MongoDB(for querying and aggregation). But in all the experiences I’ve had doing analytical work with both; Postgres easily comes out ahead. If you’re starting from a blank slate, I’d definitely recommend it over MySQL. Just update/feature addition rate along with the better community quality are enough for me to prefer Postgres over MySQL.