I have been thinking about that idea (putting database in front of operating system, i.e. applications interact with the environment through database) for a while now, I think it's really neat.

Another good reason is testing and verification. Your application becomes decoupled from the OS resources and you can verify what it does to them by comparing the database. Because the OS state is managed by database, it is formally described and you can verify what your application does to it. In some sense, it's an implementation of functional core/imperative shell idea.

Also, because DBs are transactional, transactions will become a first-class concept in your operating system. Which is something that's long overdue for applications (although there are systems such as CICS that have been doing this for some time now).