There's simply no way possible that Apple would do something as bone-headed as placing the sensor on the back. I don't even think that Tim Cook could possibly be
that out of touch. A rear located sensor requires the unnatural holding of the phone. Almost everyone holds the phone on the sides, making the current location of the scanner very natural and easy to reach with the thumb. IMO, a sensor located on the rear would essentially require an entirely new re-programming of users in how they handle their phone.
Just not seeing it.