Read my post again... This time carefully... And try to fathom it... I know it's a difficult task... Please try...
No matter how much you say, give option does not lead to fragmentation... Based on Apple's controlled system, fragmentation will never reach to Android level. You need worry more about feature fragmentation.
Three people will probably stay in iOS 6:
1. One with jailbreak or just does not want upgrade due to whatever the reason. You will not see these people upgrade. They are also likely to demand way to return iOS 6 if they ever upgrade, if there is no way, they won't even upgrade at all.
2. People who upgraded to iOS 7, for whatever the reason they might have, they want to return iOS 6. Some of these people returned to iOS 6 during that extremely short allowance period. Some people stuck with iOS 7 forever. They might sell their iOS 7 device and swap with iOS 6 devices. OR they get extremely annoyed went to Android.
3. You get the people who are indifferent between iOS 6 and iOS 7. No matter how much you loved iOS 7, there are signification population just don't care. So when they upgrade to iOS 7, they probably will stay with iOS 7 or they might downgrade for whatever the reason.
So when you open up the ability to downgrade, only these type of people might be downgrade. If iOS 7 is such great OS, then probably people in 3rd category won't downgrade.
By the way, even if you force everyone upgrade to current OS given their device capable running iOS 7, you still get huge number of people with older device and older OS.
If iOS 7 is really that good, then you should not worry about extremely large number of population stay in iOS 6. AND by the way, iOS 7 really isn't huge change from iOS 6. Still same old stuff from 2007 with new UI and Control Centre...