I'm in the "home button is going away on iPhone 7" camp.
I think Apple are preparing us for the home button removal, or certainly massively reduced bezels and a different kind of home button.
Apple refer to it as the "Touch ID sensor" on the diagram on their 6s tech specs page now. They call it a "home/Touch ID sensor" on the 6.
Small thing, but potentially significant.
Multitasking can be activated using 3D Touch now. One less use for a home button.
I can't think of a non-home button solution for reachability right now, but I'm sure Apple will.
It's like how they gave us Touch ID on the 5s, to prepare us for Apple Pay on the 6.
They gave us swipe to go back, to prepare us for bigger screens.
Touch ID can be embedded in the screen, or in the bezel, or somewhere else.