Apple's business, if you look at where they now get their revenues from, is all about phones. How's that sustainable? Short answer, it isn't really. They might have the fashion devices but they only have one of them and it's on a fairly slow refresh cycle. If any of HTC, Samsung, LG or any other of the Android vendors come up with the Next Big Thing in technology fashion, Apple don't have another card to play and the idea that one company in such a competitive market can lead both fashion and technology in perpetuity simply isn't credible. Yet with Apple the only player in their ecosystem, if they stumble the whole ecosystem stalls. If one Android vendor stumbles, another takes their place, the ecosystem keeps going and the vendor has the opportunity to catch up at their next release.
Apple might currently have the biggest selection of apps, but with Android fast closing in market share (indeed, looking likely to overtake) and some vendors deciding Apple's policies are too restrictive and switching away from the iOS platform, how long will that stay the case? Once iOS is no longer the dominant platform for app vendors, their more restrictive policies will start seeing the market fall behind its competitors and, just like MacOS, iOS slowly becomes a ghetto with limited software availability.
So what are the alternatives? True low-end iPhones? (I can't see a 2+ year old design doing well for very long against newer models in such a fashion-conscious sector.) Not great for Apple; more design and manufacturing expense, more complex supply chains, lower margins and the first-mover profits that Apple are currently enjoying are gone. Quite simply I don't think there's any precedent to suggest it's possible for Apple's current profitability from the iPhone to be maintained, and very little reason to believe its market position is sustainable.