The thing is, I don't have hard numbers for this, but from what I know (and it makes sense, given my personal experience with people and desktop OSs, car brands, clothing brands, etc.), regular people don't switch that much between mobile platforms.
I'm pretty sure Android is growing faster than iOS, worldwide. And most of the users that get entry level or mid range level Android phones won't "upgrade" to iOS, they will just get better Android phones, since they already know the system.
Barring some super successful entry level iOS device [1] being introduced by Apple, which is unlikely, Android's market share will only go up. Apple will still be super profitable, though.
[1] And that won't happen, as Linus from Linus Tech Tips put it: Apple is jewelry and people don't buy cheap jewelry :)