JSON storage is only half the story, the other half is ease of scaling. Scaling Postgres is not simple, and given enough of the right kind of data, I could see

JSON storage is only half the story, the other half is ease of scaling. Scaling Postgres is not simple, and given enough of the right kind of data, I could see why someone might chose MongoDB. But for quickly iterating on a schema for a new project, Postgres's JSON support is definitely the best of both worlds.