Also, let's be honest, outside of some loud privacy concerned voices, most people don't care and they will prefer to scan their faces with the phone they have at home which is an action they are already used to do than to spend money to buy some nerdy device.

When I was 8 or 9, my Father would often give me 50p to go down to the nearby pub to get him 20 cigarettes and a half-pound bar of chocolate, which came to 48p. Always "and where's my change then?" when I returned. Tight git.

The same can happen with any type of authentication, unless you are ok with someone pointing a webcam and authenticating you non stop.