Multiple Backs on 35mm DSLR
That's an interesting idea. I suspect that Nikon might be shooting itself, so to speak, in the competitive foot with its other camera bodies, particularly among pros who could economize on their equipment this way.
I remember that in my Nikon F days many years ago, I suggested several times that a replaceable back design would enable pros to carry both color and monochrome film cartridges or cassettes and even in several film speeds for quick interchangeability. I soon found that pros typically carried two or more bodies instead anyway, either for film changes or for lens changes, and that way Nikon could sell more bodies.
By the way, Zeiss (the real Carl Zeiss West German company) produced its flagship and final German-made Contarex SLR with interchangeable film backs. The camera weighed probably as much or more than a D700 and was a typically extremely-high quality high-end camera but, at its price, and size, could not compete against the pro Nikons and Canons of its day. There are often one or two available on EBay from time to time and have more collector value than functional value. (I just saw one for $750, body only, but no bids so far. For sheer quality (especially with German-made Zeiss lenses), they are among the best film cameras ever made, but there weight and general ungainliness made them non-competitive dinosaurs.