After reading a few posts.
What Apple serve is not only us customers, if any.
Ok, and even though there is a huge install base of iPad 2, but also large amount of them don't even bother upgrading them to the latest iOS. They are maybe at iOS 7, 8, or even 6 if not iOS 5. Why? Because their productivity app runs perfectly fine on older system. I have seen so many iPad mini in my school still using iOS 7, yet I feel no need for them to upgrade to even iOS 8. Not all guys need the latest and the greatest.
iOS 9 on A5 is already super slow. I can clearly feel that. And also unfortunately most customers don't care about "optimization" thingy but just know to complain when their devices run iOS 9 slowly. Plus, as iOS gets bigger and adds more features, there is a threshold preventing those engineers to optimize iOS 10 in order to run on A5, I believe.
Apple is not Microsoft. Microsoft needs windows to run on as many devices as possible so they need to spend a lot on optimising Windows 10 so that it can run on even 6 year old hardware. Apple don't need to. They want to sell more hardware, latest and "greatest".
So I feel frustrated because of apple style inconsistency again, but not the fact A5 is abandoned at iOS 10 stage.