I don't think the slowdown is because of little support < iOS 6, I think the slowdown is because they can't make a phone so much better than the current best that people will want to buy it.They're gonna' slow because they cannot physically sell an iPhone to every man, woman, and child out there. If iOS's switch to a flat aesthetic and dropping support for old models was the reason for their downfall we wouldn't have seen quarter after quarter of massive growth for the past few years.
iOS 7 was released coming up on three years ago. Why was there such a lag if it's the causal factor?
About the only things I can see them adding that'll make it better is a usable edge where the screen wraps over one edge. Battery live going up by a good amount (even if it has to be a bit thicker. Much better front camera, they've always shorted the front camera, yet people seem to use it just as much, if not more.
After that, what more do people need to have in order to upgrade.
The lack of support for < iOS 6, really hurts Apple in the poor nations. Poor nations have excellent choices in Android, no choices in Apple, even on the used market. Apple could have supported iOS 6 and had a strong foothold in the software/services in poor countries that could use the 100's of millions of < iOS6 devices.