I don't see the point in this really, wouldn't they be competing against themselves if they did this? I doubt that many people would care the difference between plastic and metal/glass, most people put their phones into a case anyway.
I think they have a good system going here with having previous generation iPhones be a lower price to make having an iPhone still affordable and ranking on price depending on generation, rather than materials that the shell is made out of but other than that it's pretty much identical..
I think that would just hurt a lot of their sales.