When Tim Cook said that about fiscal 2016, I think it was indicative of former iPhone users switching BACK from Android, after Apple finally gave them a choice of larger displays.
So yes, iOS was the appeal, but I think mostly to those who were already used to it and missed it.
True, it still shows that Apple can still has appeal, if people switched back because they finally made larger screened phones. I think if the iPhone 8 is a big upgrade it will cause people to upgrade their contracts, or those who like myself buy our phones outright. Obviously we will have to wait and see in September.