People like a new toy
I think the reason the lack of changes to iOS could start to impact Apple is because people like new toys.
Personally I'm of the same view as you on this, I don't want change unless its shows a real improvement, but then I tend to keep my kit (be it phones, laptops, cars) until they stop working properly or the company makes me have a new one.
However a lot of people like to buy the latest stuff, they like to have a new toy, and there's nothing wrong with that. However if they see the last few versions of their current choice is basically the same as the last one, with just a few improvements then they will naturally start to look at what else is there to play with.
Maybe I'm wrong but an awful lot of people I know like to play with something new, even if they eventually decide they don't like it.