I agree that fragmentation isn't an issue comparatively speaking, for what's proposed with iOS here (an additional PPI device with the same resolution and aspect ratio), but I disagree that the Wintel model fits the same as the mobile device model. The nature of touch screens alone makes a much larger "fragmentation" difference than you would get on a pure command line or mouse/keyboard type of interface. And even though the PC market survived in spite of "fragmentation", that doesn't mean that it wouldn't have been even more successful in certain ways without it.
If you are any good at GUI programming, then different screen sizes is not an issue. If you only are able to write code for a specific number of pixels, then you should find something else to do.
Without IBM PC clones, the IBM PC had not been as successful as it is. IBM PC clones led to domination but also fragmentation. Apple, on the other hand, chose control and conformity.
Looking at the iPhone vs phones running Android, it's the same all over again, with the difference that Apple got a big head start this time. Any company can make a smartphone with Android on it, but only Apple can make a phone with iOS on it.