Very worrisome — a canary-in-the-coal-mine indicator that casual users no longer trust Apple with major iOS updates. Last year the number for iOS 7 adoption was in the 70s in October, which was a faster adoption rate than iOS 6 the year prior.
[Here’s my take. Lots of devices don’t have the room necessary to complete the update, and folks haven’t made time to do anything about it yet. My son’s device is a perfect example, 16G, totally maxed out. I haven’t had time to check that it’s backed up, wipe it, update, and then reload. It’s gonna wait…]