"Plan to throw one away; you will anyway."
The quote is Fred Brooks from his book "Mythical Man Month." It seems to me the author's post reflects that quote. Granted they didn't throw the entirety of every application out, but arguably it's an entirely different application now. I would even wager there's months of refactorings ahead of them as they take advantage of Postgresql's features.
The author of the comment I'm replying to seems to advocate a different approach where you choose tech and techniques that are good for short-term but maybe not long-term. This way you have planned technical debt plus unplanned technical debt. Also the comment author seems to be missing one of the post author's main points, which is that MongoDB's schemalessness is awkward even in the early stages of the app, and this is worked around by a less-than-ideal technique of moving the schema from the database layer to the application layer.