I doubt we're going to be moving on from Blu-Rays that fast. After all, has DVD lasted us two whole generations now.
Blu rays have a theoretical 200GB maximum with the max amount of layers. However there has been no need to produce those disk yet. If Sony wasnt just full of it when they said that we should be fine for awhile yet still.
What are they going to fill the disc with anyway? More streaming video data that's just less compressed to act as filler?
In-game rendering saves alot of space......
One thing they can do is stop using cg cutscenes. We've reached a point where in game graphics will do just fine.
Most games this gen alone never came close to filling the blu-ray disk so I highly doubt we'd need bigger ones.