If it ain't broke, that's success, not failure.
I've seen a few complaints lately about Apple's so-called 'failure to innovate' their iOS user interface. Sometimes with this picture:
So I thought I'd put together a picture too:
If you get a UI right in the beginning, you can refine it without ever needing to completely reinvent it. It's not some mythical 'paradox of success', nor the loss of Steve Jobs, nor the inability to keep up with MS or Google that keeps Apple from making grand changes to its UI.
It's the simple fact that it already works properly.
If you need to totally overhaul your UI in order to improve it, that just shows you've been doing it wrong.