re:
This same App fragmentation happened back when the iPhone 5 came out. Up to a year later, some apps still hadn't been updated. Personally, I think Apple should put App store restrictions in place that forces developers to not be lazy! Like, if an App hasn't been updated in X amount of years, it gets pulled from the App Store until the developer submits an update. I think this would solve the various device fragmentation issues due to screen size and whatnot.
One thing that I DO like about this cycle of screen size changes is *most* apps will automatically scale in a "zoomed" format to fit the bigger screen. Back on the 5 when it came out, non-updated apps had this ugly black border around them.
Just my two pennies.