You're mixing up IBM z (mainframes, z/OS) with IBM i (AS/400).
The latter has a single level storage system where disks and memory are part of single address space, and a variant of DB2 database is part of core OS thus making it common to use SQL for many things people wouldn't expect.
AFAIK it still uses a classical file system, just with a DQL system layered on top. I am talking about completely scrapping the idea of file systems, folders, the lot of it. A central database that stores all blobs, configurations, info, and other metadata (modified, created, revision history, diffs, keys, tags, and authority). This would enable real time distributed scalable systems, potentially immutable backups, and a lot more (such as fully auditable data transfers, messaging, access, and more for rapid debugging).
If an operationg system was built today for tomorrow it would certainly be built on/as a database.
Z doesn't really have files or databases, but rather an emulation of variable-length records contained within a relatively fixed allocation on disk. Mainframe DASDs also have a much richer interface than just read/write a given block; for example, you can scan for records within a track that have a given prefix
