The main benefit that I see in writing for the filesystem first is that you _can_ defer the decision of database type and structure until it actually becomes im

The main benefit that I see in writing for the filesystem first is that you _can_ defer the decision of database type and structure until it actually becomes important, then you can make that decision for each component of the system and swap them out independently. The typical process, for me at least, has been to write out all the requirements of an app, then to start building migrations, models, and tests. Flipping this around has made me think about each layer differently and it _does_ blow minds when you show a simple "jack" that can be swapped out for any type of database at any time without affecting anything else.