Who said return ? Why not both ?These rumors have mixed up so much. Apple doesn't return to Touch ID after saying Face ID is thousands times more secure.
FaceID is more secure, but the combination would be even better. Use FaceID for unlock (only unlock when looking at the phone) and with full-screen touchID you could do continuous authentication so that even after unlocking it simply wouldn't respond to someone else's touch (or at least lock within a few touches).
You could also do cool stuff like detecting which finger touches the screen, so maybe 3DTouch can be replaced with using a different finger for contextual actions.Click with your index finger and 'right-click' with your middel finger, for example.