People like having their movies in some sort of physical form. I think that while the number of movies you can put on a disc may change, the basic medium (a small, flat, round disc that you put in a drive) is not likely to change any time soon. Streaming is all very well for watching your weekly TV show, but when you're buying a whole season of the same TV show, you want it on DVD. That way you can play it anywhere, not just on a computer that has a high speed connection and the proper software installed.
I don't forsee DVD being replaced.