Perhaps they just don't see DVD's dying off so quickly. I think people are going to want as much time with DVD as possible, to build up their collections, just like how they did with VHS.
If DVD lasts another 3 years for example, a lot more people should theoretically be able to get at least 8mb broadband. By then, we'd probably have 16mb or 32mb as maximum. Heck, we may be able to have a high enough bitrate to stream and record at the same time (in essence, streaming it as it's being downloaded to a media device, suitable for later playback).
I don't think it's 20 years away, but I don't see it coming in til at least 2010 either.
Also, another advantage is easy portability. If you can fit all your video collection on a single device (or a few devices), you'll be able to literally take your entire collection with you.
I always did think that the HD format wars did actually depend on the failure of digital downloads.