I think the difference there is that all of those are just storage devices - you can't just look at a DVD and see the movie, you already need a playback device

I think the difference there is that all of those are just storage devices - you can't just look at a DVD and see the movie, you already need a playback device in order to watch it. Given that, playing it back on the same device without having any external storage media is basically just cutting out the middleman. Introducing a playback device (such as going from book -> Kindle + ebook) is quite different, I think.