I'm not sure how a slower adoption of the new iOS version is proof that anyone is a failure. It's a real challenge to satisfy the needs of that many users but I think Apple did a great job with iOS 11.
I don't understand what the issues are with iOS 11. It's the best version so far, IMO. So many improvements for the better. I love that (on the iPad, at least) you can put more than four items in the dock. Wish you could on the iPhone too. Control Center is awesome. The "Files" app is a step in the right direction of gaining access to the filesystem.
I would like to see it adopt some more features that Android has, such as Smart Lock (where when certain bluetooth devices are paired, the screen stays unlocked). But it's made huge progress in the last few years.