I think this metric is totally meaningless. Like others have already stated, Apple persistently nags you to update until you do. If you have to restore your IOS device for any reason, you have to restore to the latest IOS version. They make it next to impossible to go back. So Apple bragging about something they are forcing you to do and then comparing their numbers to the competition (who doesn't force you) seems disingenuous to me. The larger question is, why does this even matter to us? Does this measurement somehow make Apple's products better because it has forced more user to the latest OS? This just makes me yawn.
How is it useless? Have you ever made an app? How is knowing what features your users can be expected to have useless? I can’t imagine a decide where the developers are blindly making apps not knowing what the users will be able to do with them. It’s good to know on iOS if make an app almost everyone using it will be on similar footing... vs Android that gets more fragmented every year. People say the metric is useless and then ask why many apps are iPhone first... it’s because of metrics like this. It’s useless to you... not useless to everyone. If you want your phone to continue working stuff like this is important.... not a hard concept to grasp.