There is no way Apple is going back to Touch ID.

Kuo: If Face ID is ‘Well Received,’ It’s Likely 2018 iPhones Will Fully Adopt TrueDepth Camera

A key question on the minds of many investors is whether the new iPhone models to be launched in 2018 will support Touch ID (fingerprint recognition). We believe the key factors are: (1) whether or not Face ID (facial recognition) of iPhone X provide a positive user experience; and (2) the technical issues that Apple (US) will have to address with an under-display fingerprint solution. We believe Apple will replace the existing Home button-based Touch ID with the under-display solution for higher screen-to-body ratios, if it decides to bring back Touch ID.