The other factor is that the modern — that is to say post-iPhone — smartphone market is 11 years old. It’s maturing, and in a mature market people replace devices less frequently.
I have a lot more thoughts on the bad downgrade to the coming quarter that Apple just announced, but my thoughts are not that important nor interesting. However, the anecdotes that I see bear out what Gruber states above: people do not upgrade nearly as often anymore. Coupled with he rise of the discount carriers in the USA and Apple not really having a phone to compete at the lowest levels and you are going to continue to see bouncing sales.
It is possible that Apple’s revenues are going to flatten out quite a bit. You will not see as high of highs, but the coming quarters will be even more interesting from the perspective of what iPhones sales are going to look like going forward.
There are, obviously, options for Apple to drum up sales numbers but the better question is what this new, hopefully humbler, Apple is going to look like going forward.