My wife has an iPhone 4. She's not a technical person; she just wants a phone and some apps. She has no interest in upgrading.
Moreover, her iPhone 4 was the first smartphone in our family. Since she got her phone (and stuck with it), I've gone through at least four or five phones -- always because they broke, not because I was interested in upgrading.
So I'm guessing that while there are lots of Apple gotta-have-the-latest people out there, most people just want a phone that works. And if Apple's hardware and software quality are good (or even half-good), then people will just keep using their old phones.
Which means that Apple will continue to sell phones, but the number of people who will buy or upgrade each year will shrink, at least to some degree.
It should also be noted that Apple is still very profitable! It's just less profitable than it was last year.