on Demand is the future anyway. As long as games like GTA IV or Gears of War2 fit on one DVD, I do not see the need to change the format. In 2012 BluRay will be more than enough.
300GB per disc is insane and nobody needs this. Think about it, my entire game collection could fit on one disc. That is a lot of space wasted and I do not want to pay for that wasted space.
Next-gen optical storage will be driven by the Movie and multimedia industry as it happened with BluRay and DVD. And they will need 300-500GB discs for prosumers interested into creation of 4k content, that will be made accessible by the likes of RED in the next 2 years.
On demand has its future, but won't cut it at really high bandwidths: we're talking 200+Mbit/s for the compressed video track only, with the best wavelet compression available. Thus it will be ok for some uses (Tv shows, rentals) but not for full quality when 4k hometheater systems will become widespread, let's say 5 years from now. Right now I can't see the net infrastructure consistenly keeping pace with these bandwidth demands and a good worldwide coverage in the next 5 years.
In all cases when you'll want to keep a local stored copy of the data, the optical media still seem the safer way to do so (HDD storage failure rates increase with size, ask any enterprise about the headaches of backup systems)
Consoles will trail 2-3 years after, they don't need as much space right now, as I don't see widespread adoption of anything better than 1080p for a while.