Some folks have told me that it is unfair to compare iOS and Android on this metric because iOS is effectively just three devices (iPod Touch, iPad, iPhone), whereas Android is a multi-manufacturer ecosystem with dozens of devices. This line of thinking is extremely frustrating to me. Developers and users don’t care that the two platforms aren’t the same. Users want the most recent features and security updates, and will demand them either directly (by complaining) or indirectly (by making a different purchasing decision), and developers want a unified base to minimize testing. Android apologists can list off the differences between the two all day long but it doesn’t change the fact that more versions with smaller share is worse for, at the very least, developers and users.
I expect major-new-version adoption rates for iOS to get even better now that Apple has implemented over-the-air software updates.