I have to differ a bit with this view. Speaking as a collector of primarily TV series on disc, I can safely say that there are probably hundreds of series that will never see a physical release beyond DVD. So the continuation of DVD sales is quite important to me, at least until I can complete...
Not sure if the practice has stopped, but I have seen many DVDs that have trailers for upcoming releases. Disney titles for sure.
Plus, the nostalgia value probably varies by generation. You can use your DVD player and early LCD flatscreen and feel like you're watching in 2004.