With the Sidekick, PalmOS, and then Material Design, Matías Duarte has been consistently at the forefront of mobile UI design.
Jony Ive is probably the most brilliant consumer electronics hardware designer in the world, but I have yet to see any indication that he has any idea how to lead a platform UI design project. I had issues with Scott Forstall's style, but he was at least leading the charge somewhere.
In my opinion, force click, Siri, 3D wallpapers, etc, have not been major coups in terms of UI finesse. At best Apple is in a holding pattern right now, I haven't seen any indication that they're pushing the state of the art outside of the hardware. The animations keep getting flashier and smoother, but that's not really UI design.
I thought Ive's strong understanding of design methodology would be enough, but I think maybe software design requires a different way of thinking. I don't know.