The MacBook Pro Retina 13" is almost the same weight and size as a MacBook Air 13". It no longer makes sense to keep the Air around as a "lighter" alternative.
IMHO, both the Air and the non-retina-Pro are going away. Keeping just the MacBook and the retina-Pro is what makes sense the most (the rest are just cheaper alternatives, and Apple doesn't generally do cheap).
Also even if the current lineup is not perfectly orthogonal because the purpose of the Air is less clear than it used to be, this has happened before when new lines are introduced. What leads you to assert that they are 'rapidly falling away' from anything, rather than just in a transition period with the new line?
I like having more choices, personally, but Apple used to have a much more streamlined product offering.
I don't agree that 2 sizes of iPhone is any different from 2 sizes of laptop or two sizes of iPad.