But that's exactly what Apple already did with the iPad mini.
We're only talking a slight increase in pixel density here. Much less, in fact, than the difference between the iPad and iPad mini.
No, read again my post. You can't expect to use comfortably current iOS apps with a sensibly higher pixel density than the current 326 ppi. You quote only one line.
It worked with the 9.7" -> 7.9" transition on iPads only because Apple exactly chose 163/326 ppi for the mini (coming from 132/264 ppi).
10% higher pixel density mean everything will display 10% smaller and touch target will all be 10% smaller. That doesn't sound like a good plan.
You can't compare with the two iPads situation: every apps were already written wrt to HIG for a 163/326 ppi screen but used until the mini only on 132/264 ppi devices.
And here you're going with a larger screen, not a smaller one.
Besides, legacy apps will presumably just be scaled up anyway - so their touch targets will be bigger than on the 5s.
Modern apps can use autolayout, dynamic type, etc to present the appropriate sizes of text and controls for each display.
Too complicated.
You'd have now with new pixel densities (you had 326, and you now introduce new ones on the 4.7" and 5.5") to redefine the HIG and minimal touch target, etc. Having to take the new 360 ppi as new reference, you'd have not 44 px but 48 px as touch minimal target, would design text and bitmaps accordingly to the new pixel density, etc. And then, it all would look like ass on the 4" with everything getting bigger and would lose crucial space for data on the smaller size (people will still use and buy 4" iPhone next year. They'll even still be the majority of the iPhone installed base).
You'd lose reusability of old resources although you stay at 2x, that's not really a great idea.
It's always far simpler to support a slight zoom in (and do nothing) than a zoom out. Here, all signs point to a 4.7" iPhone with similar or equal pixel density as now.