Fragmentation implies "fragments". iOS 6 has a 93% install base. An ever-shrinking 7% isn't much of a fragment to worry about. iOS 7, due to it's new shiny factor, will likely see an even faster adoption than iOS 6.
It really helped that Apple introduced over-the-air updates and PC Free (activation without a computer) in iOS 5.
iOS 7 introduces automatic updates, but I don’t know whether that’s only for apps or for the OS as well. If both, then versions past iOS 7.0.0 should have an even faster adoption rate.
