The biggest thing that Ahrendts did was merge the online and physical stores. People forget that before she came around there was very little overlap. If you bought something online and you wanted to return it you had to ship it back instead of going to a store. It was one of the things that Cook wanted to get done by retail.
Katie Cotton’s departure was in no doubt at least partly to do with her health. She died not too long after leaving Apple.
It also isn’t clear to me how much Phil Schiller was “sidelined” vs him just wanting to retire.