DVD is also playable on BD players where (obviously) VHS tapes were not playable on DVD players. There are a lot of low volume niche movie productions who's home video sales might justify a DVD release but not the higher production costs of Blu-Ray. Movies like Waiting For Superman come to mind. Look how many years it took for an old favorite with a nice sized fan base like Forbidden Planet (1956) to make it to BD.
I ask that you not fall into the trap.
The trap I'm referring to is mistaking today's prices for authoring, producing, and releasing any title on Blu-ray to remain stagnant. While DVD is cheap, and Blu-ray is more expensive, you can bet that in a number of years - perhaps just 3 or 4, it may hold almost zero cost benefit to release a film on DVD, and may actually hurt sales if there is not a Blu-ray version available.
Forbidden Planet? Heck, Star Wars is coming out earlier on Blu-ray than it did on DVD. At this point, older films often need to be remastered to even come out in HD and look good. So, I don't think we can use any one film (especially older films) as any type of basis.
New films - when they are all HD, and are all showing up on Blu-ray, across the board, are going to continue to drive BD forward. The technologies will be refind, and BD-Rs will drop in price more and more. Before you know it, you will buy a 10 pack for $5 and AOL will be sending you BDs to use as coasters.
Really though, the fact that DVDs do play in BD players is one of the key reasons DVD will hang on for so long. Yet, at some point, studios will just recognize that DVD sales aren't keeping pace and aren't making them a lot of money (or any money?) and that is when DVD will be on it's final tour.