Simone Rizzo has a good post on iOS and the progressive demotion of buttons, arguing that there's a reason Apple has started adding gestures to perform actions that used to be exclusively associated with buttons.
Assuming I’m right in spotting this trend of a) removing the need for buttons to interact with iOS or b) removing the need for the buttons to be on a specific point on the screen, the next obvious question is why. Why would Apple move away from an admittedly easy way of interacting with iOS, one that arguably sustained the initial success of the iPhone and iPad?
The addition of new gestures for key tasks (particularly navigation in iOS 7 and one-handed picture sharing in iOS 8's Messages) mixes well with iOS' new design structure, but I, too, believe that it signals upcoming bigger devices. (We'll see in September, I guess.)
The trade-off with relying on gestures too much is that, without clear borders, shadows, or realistic elements that indicate interactivity and state for buttons, the overall experience can be confusing and the interface difficult to use. A year later, I find myself occasionally struggling to make out buttons from labels, stopping my finger for a fraction of a second as I wonder “Is this a button?”. In other cases, I slow down my swipe because I can't remember if the app I'm in supports that Back gesture. I don't think I'm alone in this, but it's undeniable that gestures – whether suggested (like the Camera shortcut in the Lock screen) or implied – are becoming more prominent in iOS.
An OS that doesn't rely on buttons in fixed positions should make for a better experience when using a larger device. The issue with “button” design in iOS is still a hot topic in design circles, and I imagine that gesture discoverability will be as important if Apple is really working on a bigger iPhone. Perhaps some sort of guide to discover features could help.