Why would you go from a single step of resting your finger on the Touch ID to two steps of Touch ID and then pressing the home button to unlock the phone? The only reason I can think of doing away with slide to unlock now is the removal of the home button and bottom bezel with the shifting of the Touch ID to the back of the phone next year. The Touch ID will be just a non button sensor at the back of the phone and one will have to press anywhere of the display in front (think Magic Trackpad) and then touch the fingerprint scanner behind to unlock the phone. Else why fix what was not broken.
My take is that the entire screen will become one giant touch-ID sensor.
Think about it. One criticism of the new lock screen is that anyone can access it. What if with the next iPhone, force-touching a lock-screen notification also authenticates your fingerprint? If successful, the notification expands as normal, allowing you to interact with it. If you are not the owner of the phone, the process fails. Convenience and security, without compromising either.
Putting the fingerprint sensor at the back of the phone just brings with it its own share of inconveniences (like being unable to unlock your phone while it is resting on a table).
