I emphatically disagree on the photo app example. The only folders I use for my photos is by date and the only reason I do so is because of the limitations of the design of the computer on which those files are stored. Photos do not particularly lend themselves to hierarchies - one person's "Family and Friends" grouping might be arbitrary to someone else, that's why tags came along - they add information instead of putting items in silos.

I hope what makes the traditional filesystem obsolete is something with better support for non-treeish data, including tags.

A tag based file systems are equivalent to a hierarchy with symlinks (or directory hard links). If you don't allow tagging of tags, it's equivalent to restricting your hierarchy to one level.

Tags and directory DAGs are the same.

True. Maybe we just need better interfaces for managing multiply-referenced files.