A hierarchy (i.e. a file system) is for many people the most natural way of classifying documents. There are other ways including tagging and search, and maybe those will become powerful and second nature, but right now classification like Linnaeus makes sense when you are dealing with lots of files.

I saw that iCloud file storage will support folders like the iPhone home screen, but I have no idea if they will support nested folders. This isn't only the case with nerds and geeks. My mother, about as computer phobic as they come, is a minister, and she saves all her sermons for future reuse. They are categorized by specific topic using folders. Also related documents from different applications can use folders to be organized together.

There are a lot of features of filesystems that we take for granted, abstracting it away without providing a usable alternative is problematic.