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.