As soon as you run into the need to move content seamlessly between programs, devices, and services--in ways the makers didn't necessarily imagine--you need some kind of universal means of interchange. Something that is infinitely flexible and controlled by the user. As it turns out, we've had this for a while: files.
Now, you can ask for tweaks to the way the filesystem is used. Want the primary interface model to be searching, rather than browsing through folders? Ok, that's great. Things like Spotlight are already a step in that direction, and they can be enhanced. Want tagging? Sure! While I don't see it heavily integrated into current operating systems, I'd imagine it would be very doable.
But none of this is incompatible with the current filesystem model. It would just be a layer on top.
>The iPhone, when connected to the computer, can be accessed like any other filesystem. You move files in and out of a folder called iPhone/Music.
People have problems with that. You don't, but it's a big issue to a lot of people.
How about this, they turn on iTunes all their iTunes music is available. They turn on music app on their iPhones, and all their music available (streamed or downloaded). Every song they buy, is automatically synced to all their devices.
Which is easier? Which one requires you to understand "syncing", or file systems, or copy-pasting vs. moving, or a thousand other little things that you need to have a handle on, just to listen to music on your devices.