Previous attempts to do away with directory-based filesystems and go with a sea of tagged documents and a metadata database have crashed on the rocks of low disk performance. But those ideas are good in principle, they just weren't appropriate for actual hardware.
They were always a terrible idea because they don't scale in the human mind. For a music collection you can just about deal with artist name, album name, song name... But even when it comes to things like "genera" how many people can remember if a particular song they want to hear counts a pop, or rock, or soft rock, or maybe it was prog-rock, or is that "prog rock" or "progrock"?
It gets worse for documents. With a folder system you can drill down. It serves as a memory aid. With tags you need to search and sift through search results unless you can remember the name of that particular thing you needed, or some other fairly unique identifier. I'd contend that tagging is more effort than organizing in folders too, especially if you want to change tags in bulk without separating collections of related documents accidentally.
There are ways to reduce these problems with fuzzy search terms, hierarchical tags and the like, but they are all just lame attempts to polish a turd.
The problem of navigating a music collection is already solved at the application level by various apps. Any file system will do fine. You're not going to have more than 10-100 million music files on a system since that's about what humanity has created so far, so it's a fairly well bounded problem.
Innovation at the OS level should probably focus on problems where there is no upper limit to how many files you could realistically want to store and search.