The only big change under Cook has been screen size amd the further he goes increasing the size the lesser he leaves for the smaller screen size. The Plus got the better display. Plus stomps the small size iPhone in battery life. The plus can be faster than the smaller phone in tasks. And now we have a 6 inch phone which gets all that 3D sensing tech, integrated home button, OLED display and better cameras and overall performance. The 4.7 inch phone is quite literally in the dust at this point. It doesn't even have dual cameras or 3 gigs of ram and will be stuck on a 750P LCD while the iPhone 8 is on a 1600P curved edge OLED
Here is how it will go down. Many have not upgraded their phones from the 6 due to lack of notable features which is what the 8 brings to the table. It has everything I have wanted in an iPhone . But at $1000 while records will be broken as the ASP gets a huge boost this isn't going to be sustainable as not many are going to spend over 1K every year on a phone. Once I get that OLED display I am not going to keep upgrading every year as its costing almost as much as a high end Windows ultra book so long term is what the question is
The "only big change" under Cook is quite the understatement. That only big change catapulted Apple forwarded. And in making that change, he made a decision about how to move forward with the product lines. Plus gets better camera, battery(well duh, the case is bigger) and screen, everything else is the same.
The only reason the plus can be faster, is working on a larger screen is faster to working on a smaller screen. That doesn't mean all the masses want a bigger screen. Plenty have opined here on MR, their preference for a smaller screen.
Many also have upgraded their phones and even Cook said the demand on the 7 has been stronger than they anticipated. Taking me as an example, I upgraded my 6s due what I thought a good value add (for me). I personally will have to see if the iphone "x" is worth it and my biggest concerns about the rumors are oled technology. If I could get an iphone "x" with lcd, I wouldn't hesitate in the blink of an eye for an upgrade; (provided I thought the value proposition was there).
4.7 inch phones will survive as a successful product line for apple for the foreseeable future.