Touch ID is a fundamental aspect of iPhone. It is pivotal to both a sense of security to the customer, and ease-of-use for unlocking the device and paying with Apple Pay.
Touch ID therefore, is priority #2 after the Home Button. This means that when contemplating any changes to iPhone design, the teams involved—which are comprised of highly intelligent human beings—analyze what must be done to preserve the functionality and the location of the Home Button and Touch ID. As these are priorities #1 and #2, all feature changes and design changes will only be undertaken if the functionality and location of these remain static.
Touch ID will not be located on the back of any iPhone, ever. It is poor design, an enormous compromise, and as I stated, a direct offense to priorotization.
Touch ID will not be located on the elongated power button. It has a new use being elongated of which has nothing to do with Touch ID.
Touch ID and the Home Button will remain on the bottom face of the 2017 iPhones. The Home Button will have identical functionality, ease-of-use, and feeling, but it will now be a part of a larger area known as 'Home Bar'. This will be a black bar like the Touchbar on MacBook Pro, with included Home Button functionality.