I think that we should analyze this question in different aspects:
- technical and functional
- graphical
While hopefully every new main version of the iOS brings new features and improvements (with the ususal hick-ups in the launch period), changing users' ergonomic experience is often also a major factor.
Therefore we will have users that want to revert back for
(1) having disfunctions with Wifi connections etc. (clearly related to the OS)
(2) having issues with some of the apps that they consider to be key apps for them
(3) not feeling comfortable with the new UI
Regarding the previous mentioned points, the solution could be
(1) test, test and test; and if at the end there is still some issues left, bring rapidly a corrective version.
I think that here the Apple OS developers do a better job then for other OSs; but they also have a strategic advantage by the fact that they need to comply only with a limited number of hardware (Apple products).
(2) issues with apps are clearly out of the scope of OS designers, yet are a major issue for the endusers.
If an App is maintained then likely fixes will be delivered in short time. In order to distinguish apps with support from those without support, the AppStore should remove (or block the installation of ) apps which did not have any updated version since version n-1 of the current iOS (i.e. apps that had the last version released during iOS5 should be considered risky under iOS7).
Therefore, before in the beginning of the migration to a newer iOS version a scan of the installed Apps should be done, taking users attention to such risk factors, while allowing him to decide whether to progress or not.
(3) finally the UI is the main factor of interaction between the user and the device.
In order to smooth the transition or satisfy the nostalgic, it should be possible to have themes reproducing a UI like previous versions of iOS.
Additionally one could only recommend to Apple a less "agressive" notification that a new iOS version has been released. Lots of average users just get trapped with the message that their device tells them they can get something new and do the change without many considerations, but feel trapped then.
Nonethesless users should have the possibility to revert back, even so if this also includes of reverting to earlier versions of some apps.