That would make sense if those unique UIs didn't themselves change fairly drastically every few years. I loved the older iOS devices because of the physical home button - I could give it to my grandma and say, "if you ever get lost, press this and you'll be back to home screen" - and it worked wonderfully... until they removed the physical button. On the latest iPad, you have to swipe from below the edge of the screen instead, and do it fast enough (if you do it slow you get the task switcher instead of home). Needless to say, this requires relearning.