Re: Comparing apples and....
Spend per device is a useful measure, but only to a degree. Popularity can change quickly - just ask anyone over at RIM/Blackberry.
If iPhone popularity were to fall significantly, and it would have to be a huge drop, then they'll start to lose app makers who will inevitably go where the money is.
Another problem Apple face is that, as phones become more feature filled as standard, the need for apps that are only available in the iOS ecosystem begins to fall. As that happens, so too does the extent of Apple's vendor lock-in that comes from customers not wanting to lose all their purchases.
I'm not predicting the demise of Apple, only pointing out that they will need to keep finding ways that allow customers to justify the price premium