But you don't need to horizontally scale if you use a beefy Postgres instance. Horizontal scaling is a problem created by using an inefficient DB in the first place.
I have DB clusters with masters with a terrabyte of ram. Going beyond that is a PITA with warming up cache, backups, and so on. There is a reason there's a huge industry for shared databases.
Also, query for query, Mongo isn't going to be that much slower than PG, and faster for some usage patterns...