Mongo solves it by not being ACID compliant, which is sort of a big deal. This is like comparing apples to watermelon. Essentially, mongo solves this the same way redis does, but you said redis doesn't count as a database.

Also, it is possible to horizontally scale postgres. It's not for the light of heart, but the easiest way is with read only replicas, which is pretty easy (especially with RDS), and read heavy loads are where postgres really shines, and also most apps out there.

MongoDB is ACID compliant. Yes, I know someone will post the 18 month old Jepsen report, but the fact is those issues were fixed almost immediately. The Jepsen test suite continues to be part of MongoDB's release testing.

I am strongly against treating Redis as yet another cache. For me it is a perfect database for non-critical short-lived data, that can be reproduced from more reliable databases in case of failure.