While these screen resolutions and screen densities might make 'sense' one of the problems which comes out of this for developers and users alike is that App downloads and local installs keep getting larger due to all the different assets for all the different screen resolutions which needs to be bundled into the app.
Already apps with limited functionality are approaching very large sizes due to supporting 2 iPhone and 2 iPad screen densities, adding more is going to make this problem worse. Everybody will soon need 128gb iPhones just for all the high resolution assets they aren't using.
This is aside from the problem that in reality irrespective of whatever screen resolution Apple will put out most developers will just blow up the smaller iPhone screens to fit the large one, partly out of laziness and partly for cost reasons. This just means that the buttons are going to be bigger.
Now considering this will be the most likely outcome in 99% of apps, they might as well just put a 1080p screen in both and scale the existing contents accordingly, the pixels will be so small you won't even see any distortion of the existing retina images and yes the UI elements will be bigger on the 5.5" but thats going to happen in most apps anyway.
Already apps with limited functionality are approaching very large sizes due to supporting 2 iPhone and 2 iPad screen densities, adding more is going to make this problem worse. Everybody will soon need 128gb iPhones just for all the high resolution assets they aren't using.
This is aside from the problem that in reality irrespective of whatever screen resolution Apple will put out most developers will just blow up the smaller iPhone screens to fit the large one, partly out of laziness and partly for cost reasons. This just means that the buttons are going to be bigger.
Now considering this will be the most likely outcome in 99% of apps, they might as well just put a 1080p screen in both and scale the existing contents accordingly, the pixels will be so small you won't even see any distortion of the existing retina images and yes the UI elements will be bigger on the 5.5" but thats going to happen in most apps anyway.