As we all see now, iOS 9 keeps everything from iOS 7 and 8 except San Francisco font and some icon changes. (I believe there should be other under the hood element changes though)
And in my view, I prefer this clean and a little bit bright ui. This gives me a quick impression of what a button should do. Unfortunately I still can see apps using skeuomorphism design, which looks quite outdated, except some which didn't update any more.
Design is a very subjective area, and it can differ from person to person. Sometimes two design represents two completely different understanding of an object. Apple has become a company with their own DNA. In this case, the grid homescreen, and round icons, fixed layout, is apple home screen DNA. Everyone see the home screen could say: hey, he is using an iPhone. That is a really unique point. If apple change their DNA, allow more customisation, I believe users could use icons to draw their own picture.
. This is funny, but not common.
Therefore, I think, Apple should insist on this design concept, to make all apple products unique, and elegant. Then apple must limit the customisation of user interface. We can also say this is the DNA of apple culture.