Rock and a hard place
Whilst historically it has certainly been true that Apple devices were more durable and better supported, Android has matured considerably, Google has started to take more control of software updates, and the build quality of any well chosen Android is easily a match for Apple (and usually less fragile, although Sammy seem to be missing on that one). Whether the better resale values of Apple devices hold up we'll see, but looking at the behaviour of the company, I can't see that they'll fail to react to the "loss" of revenue caused by selfish non-upgraders. They've been caught out on the battery throttling, I'd assume that they'll look to make sure that future IoS releases are somehow much slower or incompatible with older handsets ( a trick which has worked well for many Android makers).
What we're seeing is the convergence and commoditisation of handsets, and most Android makers are felling the pressure too. Under that dullard Cook, Apple seem unable to innovate their way out of this bind, and we're looking at managed decline. I suspect that the widely rumoured move to USB C will be another blow to the proposition of the unique walled garden.
The only one thing that I can think of that could save Apple would be a new battery technology that offers much greater capacity (so a week between charging for typical users), and where Apple own the technology and Android makers can't use it. With such a leap forward in the ownership experience, they'd get away with battery life of two years, and charging a serious amount to replace it. OTOH this is unicorn technology, and its battery chemistry, so not clear how Apple could find and own the technology.
A more achievable alternative would be to take the handset leasing out of the hands of networks, and offer most, maybe all of the Apple ecosystem, including a phone with a two year refresh on a continuing subscription basis. Stop selling retail iPhones for cash, just offer one monthly price for everything, and capture all the revenue bleeding to Netflix, Spotify, network operators etc. Family discount would avoid breaking the typical loyal household market. Improve the ability to cast content, major on privacy, improve the ebook experience, and look to squeeze Google and Amazon. Then offer a much improved iPhone for business experience, attacking head on the issues of security, IT policy and device management, Exchange integration, and besides the B2B customers, use that division to sell handsets to retail customers who just want a phone and don't want the "full fat" proposition. All of this is high risk, on the other hand potentially high reward. It is also achievable with Apple's technical and commercial skills - and bottomless pocket. Sadly Cook is not an innovator nor a risk taker.