SD-DVD will be here for many years to come and I'm not sure why anyone would think otherwise.
Something else to consider is electronic distribution vs physical media. Video OnDemand can make movies available without needing physical media but not necessarily the extra features that tend to come on a physical DVD release. If you're into the extra features, physical DVD releases are the obvious choice. If you're not into the extras (or not as much), physical media becomes less of an issue. Collectors will want to own physical media but I can also see collectors wanting to collect
movies they like and the medium on which that movie is stored becomes less important as long as the movie itself is readily available.
We've seen no trend of SD-DVD releases slowing down and I don't think that will happen in the foreseeable future.
Peace...