The publishers want to wipe out the 2nd hand market, so I think downloads will replace everything else, but since downloaded files tend to be much smaller than DVDs or Blurays, game developers will have to use cartrige-like development techniques if they want to fill those tiny files with lots of content. For example, I wouldn't be surprised if MIDI music makes a comeback on the next gen of consoles. I've played around with music emulators and was happy to see that the entire Final Fantasy 6 soundtrack in its cartrige form took only about 370K.