This year, we will see iPhone 7 which supposed to have big improvements over the current generation in normal conditions but next year will be 10th anniversary of Apple so they had two choices with the upcoming 7 series: they could either make big changes with iPhone 7 ( which will be congruent with their Tic-Tac cycle ) while not introducing anything new for their anniversary or they could make the big changes next year in the honor of their 10th anniversary. However, it looks like Apple chose the second option.
According to The Wall Street Journal, Apple will use a design on iPhone 7 that looks like iPhone 6S and yet it will lack the 3.5mm headphone in order to make the phone even thinner ( I wonder what would happen to the battery life, though ). However, in 2017, which is the 10th anniversary of iPhone, Apple will make huge changes including all new design using both metal and glass ( like iPhone 4 or the latest Samsung Galaxy S7 ) and new OLED display.
Honestly, I found their decision right because the 10th anniversary of the iPhone is something special as well as significant.