A filesystem as backing service is a different problem (or more to the point, a different solution). But the filesystem used to store the application runtime is

A filesystem as backing service is a different problem (or more to the point, a different solution). But the filesystem used to store the application runtime is itself ephemeral. In Heroku, when the app shuts down, its VM goes away permanently, along with any local storage, and any instance of an app can be shut down at any time.