Nor have they been keen on letting users OR devs customize the ui.
The home and widgets screen can be customized to the point you don't recognize it as iOS
> devs customize the UI
Have you used Spotify? It completely ignores Apple UI and does its own thing cross platform. If you mean let devs customize the OS' UI, why would they? UI consistency is one of Apple's core strengths (or so it was before the 26 releases).