I can't imagine a format which would be more convenient than CDs. Anything much smaller and it'd be harder keeping track of them - note the way that for DVDs of films these are actually packed in a way that is relatively bulky, no doubt because the market research has shown that people prefer something they can get hold of more easily.

I suppose there will be people who want everything stored on a hard disc, or online. But I think there'll be a continuing demand for something you can get hold of and pass around.

At some point the technology comes up with something that sticks. Books, bicycles. I think CDs, or things that look and function like CDs (eg DVDs or whtever the technology and marketting comes up with, but that's basically just a matter of packaging), are probably another example.