If your looking at a specific device - iPhone 7, then yes I think we will see a decline from the iPhone 6 launch. A few things to note, the market had massive pent up demand for a bigger screen iPhone, that demand does not exist anymore, the demand from owners is now only to upgrade to a new device, not form factor. Secondly, the market is 'spread out', in the sense of there are 3 sizes all with relevant hardware 4", 4.7", 5.5" devices, the form factors suit different people. Thirdly, people like to upgrade when things change aesthetically, why drop hundreds on a new device that looks the same, they want things to look different.
I would only personally worry if the 'total combined number' of iPhones sold started dropping significantly.
On a personal note, I have an iPhone 6 128GB, it serves me well, has Apple Pay, Touch ID, high storage, allows me to play GTA, Max Payne etc with no performance issues and it is in excellent condition. The 6s improved it with force touch, this week's 7 will improve it some more, but I'm not ready to upgrade, the cost of upgrading versus those improvements just aren't worth it..... yet. Combine those improvements over 3 years and a new aesthetic and I would bite..... so next year it is. Expect that approach to be like many others.
I predict that the end of this year and next year will all be about the Watch, I think there's a curiosity and a hidden demand, from those who want the existing one to be cheaper and those like myself who want a better device. It reminds me of when people wanted iPods but didn't want to pay high prices, they released a lower priced mini and it flew off the shelves.