Comment by cpleppert

Microsoft most assuredly did not try to replace the filesystem with a database. WinFS was a relational database service that served as a central metadata store for Longhorn. It was slow, but most importantly never provided any value. The idea was that applications would store their structured data inside SQL Server cough I mean WinFS and then somehow they would be able to easily link their data. There was a complicated system of loading application schemas to accomplish this. Of course, if applications want to share data they don't need to involve a brittle system sql database.

Nothing is stopping you from storing blobs of data inside a database and then exporting a POSIX api and calling that a new and improved filesystem. But once again its hard to see what value you get from all this complexity. A filesystem is complex enough and it doesn't have to store structured data. You generally don't want the OS to handle this complexity; you have just added another failure mode to a part of the system that you really don't want to fail.