> Which is actually why the "files and folders" metaphor is apt.

It's a starting point, but I certainly wouldn't say it's the best metaphor that there could be. The idea of subfolders just doesn't make sense in a filing cabinet analogy, because you have to consider paper size - any folder which could fit into another folder is not going to be able to contain your regularly sized documents.

That said, I can't think of a better metaphor.