I still believe optical media will reign in some form, even after downloadable content. See, if for example the PS4 or whatever had no optical drive of any form and downloading content such as games was the only choice for consumers, then the risks of that download getting lost, corrupted, deleted, etc. would be the main reason for having a backup. But unlike a simple MP3 download where you can just copy the file onto a MemStick and transport it anywhere, these downloadable content will be software-protected to protect them from piracy...
Like how on the PS3 you can't copy a game bought on PSN onto your external HDD. You have to be careful not to accidentally wipe your entire HDD or delete the file. This is where back-ups would come in handy. Even though you can ask for another copy of the game by emailing Sony, you can't keep asking them for a new copy every time something happens.
Basically, Blu-Ray and all forms of optical media will still stay alive until a smaller form, in terms of physical size, of optical media comes out.
Blu-Ray will take a while to catch on. Companies are not helping the format by releasing 'DVD players with upscaling technology' that is supposed to give you the same experience as any other HD movie or not researching in ways that they can manufacture Blu-Ray at a cheaper cost, so as to appeal to the price factor many of us have.