How come iTunes sales haven’t hit a wall?

I'd say using iTunes requires less technical acumen than keeping a Windows installation running and virus-free.

Re: expecting users to be digital archivists - you may as well ask why anyone not a museum curator buys antiques, or anyone not an archival librarian keep books. Users may not expect their digital collection to last any longer than their collection of 8-tracks.

iTunes hasn't hit a sales wall because digital music is a lot more convenient that anything that's come before it, and iTunes made digital music dead simple. Having your entire music collection in your pocket makes storing it any other way seem like a step backwards. That convenience trumps any tech worries most people have.