The reason to change is scale. If your Postgres spend is in the six digits a month range, JSONB is probably very painful for you. It does not perform very well. Additionally, you've probably spent a good amount of engineering time painstakingly sharding your database at the application level and hoping you've done it correctly. MongoDB solves these problems out of the box, among others. I have run both of these above 1M a month in spend and I would certainly choose MongoDB for this over Postgres. If you plan on never spending a lot on your database, I do like Postgres a lot, still.