I also think they should do away with the numbering, at a certain point they start to become meaningless. Years of release are a much better way for the advanced users to refer to specific models, and laymen don't care anyway. It's not like we have Macbook Air 5, alongside the iMac 12s. Instead, we have the mid-2012 Macbook Air and 2013 iMac. Alternatively, you can refer to them by "gen" number, 2nd gen or 5th-gen, etc.
Also, at a certain point, the numbers start to hold some people back. The iPhone 5S is still a very capable and very good smartphone by today's standards; it's not top of the line, but I don't shy away from recommending it for people who want a smaller screen and don't plan on doing any sort of computationally taxing tasks on it. But the fact it's a "five," when the "seven" is right around the corner, seems to put some folks off. They'd rather spend $150 on an Android of similar or worse specs but that was released within the last 6 months than an iPhone released many years ago. If Apple gets rid of the numbers, I think it would be easier to sell these folks on entry-level iPhones. It's not a "five" when there is a "seven", it's "the low-cost small-screen iPhone," and that expensive one over there is the "premium model."
I would prefer they call the different models by their screen size, rounded up or down, similar to laptops.
4" iPhone
5" iPhone
6" iPhone