You're right, there is a constraint for most rational customers:

  X is in streaming => X is not in DVD queue

The thing is, the set of stuff available for streaming is not very large compared to the set of stuff on DVD. So the applicability of the constraint is small.

And, I think all the constraint tells you is, don't suggest titles that are available on streaming for someone's DVD queue. I think affirmative suggestions are much more interesting.

To me, your idea that people's streaming preferences are "virtually identical" to their DVD preferences seems like you didn't mean it seriously. Hyperbole?

My own streaming habits are totally different from my DVD habits, not only due to limited availability of streaming titles, but also due to ingrained expectations. It's kind of like TV versus movies.