There is a good article over at CNET on how an iPhone 5s from Verizon could work on AT&T. I won't go into the details here, but suffice it to say, as carriers standardize on LTE, it is possible to make a phone that works on AT&T, Sprint, Verizon, T-Mobile, and other carriers. Indeed customers already could migrate between AT&T and T-Mobile, which use GSM SIM cards. In fact, I use an old AT&T iPhone on Consumer Cellular's service. Many of the pre-paid providers like Boost Mobile, Virgin Mobile, and Straight Talk leverage one of the major carrier's networks.
So, it could be possible, and perhaps even easy for Apple to make a single iPhone that supports any carrier. Given the ridiculous number of configurations the iPhone has, it would be so much easier to stock a single phone, with a few color and memory choices. Apple could see the light and make 32 GB standard, and cut down on memory configurations as well. I don't think they'll ever offer expandable microSD card memory, as Android phones do, but that would be another way to ship a single configuration.
When the iPhone 5s launched, even Sherlock Holmes would have had trouble finding the iPhone 5s in the color he wanted, with the memory he wanted, for the carrier he needed! That's why sites like Apple-Tracker popped up. It has since been taken down by Apple, unfortunately. I wanted a 32 B model but ended up getting a 64 GB iPhone 5s, because I wanted gold AT&T and that was all I could find. Perhaps that's in Apple's interest, but it does seem like a problem waiting to be solved.