I agree with you on the original premise of "Why would you use Mongo over Postgres?", managing two DBs is terrible. But if you replace Mongo with something like DynamoDB, I think it gets more complicated. You don't really "manage" DynamoDB as much as just define what you're trying to do, as it's a managed service. I quite enjoyed using DynamoDB for some specific tasks at work, much more than I've enjoyed PostgreSQL's JSON columns.

In an ideal world yes, you only manage one database but no such "jack of all trades" DB exists for the same reason the CAP theorem remains true.

If your Postgres JSON schema modified for new application requirements, the table is locked until existing data is copied into the new schema, requiring applications to be quiesced during schema migration.

Thats a key difference between the two and one area Mongo performs best in.