Comment by borplk

Simply speaking "Postgres for everything" is meant as a fool-proof default choice for the average person making an average app. It helps startups avoid tangling themselves with some bespoke/complex combination of Redis+Postgres+RabbitMQ+MongoDB from day 1 for their app that reaches a peak of 10 requests per second with 100 daily average users if they are lucky.

This usually happens because a junior dev wants to have fun and pad their resume while playing around with tech. Or they are insecure and want to make the "maximally proper" choice with everything so they appear to be an expert. For example they think storing any JSON or cache data in Postgres is somehow incorrect or forbidden and they must use something more specific to feel like they've made the correct choice.

In general Postgres will take people very far. Majority of companies could start with it and live with it forever. If they are lucky enough to need something else by that point hopefully they have enough money and staff to re-evaluate the stack and make changes for the future of the company.