Well, it probably won't be the last iPhone, but it certainly should be. There are only so many more features that could be added to the device before it started to get out. In fact, I'm pretty sure the iPhone 5 will seem old - no matter how great it is.
I'm not saying that Apple should drop the iPhone or stop making them, but they should at least create a new model that is distinct from the iPhone. Heck, it could just be called the Apple iPhone (insert name here) for all it matters. A new looking phone would improve Apple's sales even more as long as the device is created with the same standards as the iPhone.
A fresh appearance and new features would make the new "iPhone" an even better device. Obviously, consumers will get tired if the iPhone 10 ever comes out. For now, the Apple iPhone 5 is probably going to be a great and popular device, but it's only a matter of time before people get tired of iPhones in general in their current appearance.