Apple doesn't have fragmentation to worry about. I think the requirement is smart and necessary. Developers must keep up with new SDK and new iOS.
On the other hand, older devices, at least my iPad, doesn't work very well all the new updates. I wonder if this has anything to do with the backward compatibility, or the hardware, or just the fact iOS7 is new so updates are not optimized yet.
On the other hand, older devices, at least my iPad, doesn't work very well all the new updates. I wonder if this has anything to do with the backward compatibility, or the hardware, or just the fact iOS7 is new so updates are not optimized yet.