Have you all been following the rumors of the "10th Anniversary" iPhone? Almost all the rumors are pointing towards an edge to edge display. The home button would be gone and touch ID would be built directly into the screen itself.
I have a theory on how this will play out. I don't believe they can or will shrink the current phones down to the size of the screens they have. It's just not possible if they want to keep up and improve on the internal components and specs. They already had to remove the headphone jack to fit in a better camera and haptic engine. Too much volume inside the phone would be lost in removing the bezel for them to achieve this in a single generation.
So what will they do? Use the current phone sizes as a base line and increase screen sizes to fit the whole front. If you look at the regular iPhone it's full body is roughly
the same size as the screen on the Plus. So if they stretched the screen out cover the front of a phone around the size of the regular iPhone it would be around 5.5 inches. This means the regular iPhone would have a 5.5 inch screen (and body) moving forward. The same screen size as the current Plus. The Plus model's screen would be a new larger size we haven't seen before and almost become an iPad Mini replacement.
Here's the kicker. There are many people who want a smaller, easier to handle iPhone. It's why Apple released the SE. Well the SE would go away and be replaced with a 4.7 inch model (same as the screen size on the regular iPhone). It's brilliant because it gives those who want an easier to hold, one handed phone, the perfect handset. Larger screen with one handed use.
Tech would scale appropriately and in much the same fashion we have seen with current iPhone line up. The middle one would still be the "normal" iPhone. The Plus would still have the best camera and battery life. The smaller one (mini?) would be a little behind the others in specs due to the smaller physical size of the case and the limitations the lack of space provides. Smaller battery life, not as good camera, etc...
I suspect they won't get much thinner, if at all. No need to. As the design alone of the 2017 models would be enough to wow the public and sell them on it.
I expect full credit here if my pet theory comes true.