I always found it funny that companies will go to extreme Herculean lengths to hire the best programmers, and are incredibly fearful and paranoid that they could be making a "bad hire", and yet once hired they don't spend a second making sure engineers aren't completely running the software product off the rails and killing the company internally. The author mentions trying to rewrite Etsy's backend in Scala and Mong…
In many ways it’s an argument for MongoDB considering if you’ve built a JS based application MongoDB minimizes any additional learning and having to translate between the objects on the front end and in the backend. Non relational DBs are also easier to scale horizontally without requiring any application changes.
The OP is an argument against introducing new technology without a significant clear benefit. It’s basically saying that simply having a new tech can add significant complexity, unknown unknowns, and require much more maintenance and other costs.
So if your web application is currently running on MongoDB and it’s running well then this is an argument to stick to MongoDB instead of say migrating to postgresql going forward.