I don't see this as a problem for Apple, the iPhone has been selling very well for a number of years. Last year analysts said that the iPhone 6s would not sell as well as the 6 did, and they managed to once again break their own records, with 13 million sold in just 3 days
people (myself included) will continue to buy the iPhone because they are great phones that in my opinion (and many other peoples) are the best on the market at the moment. The only reason i didn't upgrade to the 6s was because i'm really happy with my iPhone 6 Plus, not only that but i buy my phones outright (what we in the UK call Pay As You Go) but i will more than likely be buying the iPhone 7 when it is released later this year.