If current OS's are large enough that it makes sense to put an entire DBMS under them, that sounds like one of the best arguments I've heard for microkernels since the 1980s (1970s?).
(one might imagine that a microkernel schedules cores [time] and provides block access/mapping to storage layers [space]; while the DB layers would build first locking and tables, and then transactions, on top of that; after which come applications...)
That's exactly how QNX works today, except all drivers (eg. block device access, storage space) are just userspace processes, as is the database layer. But in that case, the OS is not layered on top of the database, they're all peers.
(one might imagine that a microkernel schedules cores [time] and provides block access/mapping to storage layers [space]; while the DB layers would build first locking and tables, and then transactions, on top of that; after which come applications...)