I think if these things have a retina screen, its quite possible they will canibalise 5S sales - I mean beyond us 'fans' on here (or maybe I'm speaking for myself!), I bet a lot of people will not be over concerned about the plastic. The 3G/3GS back was great for the most part, a lot people REALLY liked it even. Same software, same screen, same screen size, and I'm really not sure people will choose to pay significantly more for the 'premium' that is the 5S i.e. a bit faster, a bit better camera and a finger print sensor (so we think anyway)..
Most consumers would pay extra for the camera, but not necessarily for the faster speed. If the finger print sensor works well, it could be helpful for a lot of people who tend to loose their phone, or it could be an easier way to get into the phone that would be worth it for a lot of people. The fact that the sales of the ipad, and the macbook pro/air are so strong, is testament that people will spend more if the quality is there.