You're putting far too much importance on the DB, when really, it's an implementation detail. It has nothing to do with your app, really, and letting your app be a slave to a schema is an anti-pattern. In the end, all a schema really is a fancier linter.
This is one of the advantages of Mongo. No schema, no longer an issue.
I'm fascinated by this perspective because for any business, it's the data that is treasured more than anything else. For example, as a bank I can afford to lose all the app code but I cannot afford to lose a record of my customers and their balances. Therefore, I would never see data as being inferior to my app. The app can be rebuilt from logic; data not so easily.
In my perspective I don't want my app to even touch the schema. That is not the app's job.
It also means that just because a dev decided that the user object no longer needs the DOB field that that field will be discarded. Even scarier, what precisely happens in those situations varies from implementation to implementation. Someone who is handling the database directly will think many many times before deleting any db column. Even then, he will take a back up. I don't see the same discipline among developers when dealing with the same data, just abstracted via an object programmatically.
This is one of the advantages of Mongo. No schema, no longer an issue.