The coolest thing about using Postgres for everything is when the database works everything works and when the database goes down it all goes down, so you get to fix nothing most days then everything all at once.

> when the database goes down it all goes down

Suppose you use PostgreSQL + Something Else instead of Just PostgreSQL, and PostgreSQL goes down: Is anything still working?

I suspect the answer is "Very little still works when the DB is down", so the opportunity cost of Just PostgreSQL is low.

Also, while it's possible that PostgreSQL still has concurrency bugs, I think for most teams the odds of hitting a concurrency bug in PostgreSQL are much lower than the odds of hitting a concurrency bug in your own complicated bespoke in-house distributed system.

Only a Sith deals in absolutes