Well, for Amazon One you have to use your cellphone (based on the photos I’ve seen).
To me, that’s less convenient than a credit card / debit card.
Gee, now what if there was a scan-able tattoo on the right hand or the forehead...............
They are also already doing the same thing with store discount/rewards cell phone apps. And half the time folks don’t know how to use the app correctly so the checker has to help. By they time they get done messing with the app it would have been faster to just count out cash and give change.