people like owning movies, and the box that goes with it.
DD is on the rise, but doesnt mean physical discs are dying. blu ray doesnt need to sell as much as dvd to be a success.
audiovideophiles won't be into DD for a while either for obvious reasons.
once hd dvd is officially dead, studios will start to push retailers to narrow the dvd selection in favor of blu ray, and basically force consumers to go blu ray. they are going to want you to buy your movies in blu ray again, just like you did from vhs to dvd.