FWIW you should never relay on Touch ID as a sole means of authorization:

>>> You must enter your passcode or password for additional security validation:

>>> after you restart your iPhone, iPad, or Mac;

>>> when more than 48 hours have passed from the last time you unlocked your device;[1]

I mean sooner or later you'd run into issues unlocking your phone via Touch ID.

1. https://support.apple.com/en-us/HT204587