Regarding Amazon Go figuring out how to accept cash: any business technologically competent to run a cashless and register-less store surely has everything on camera as well as digital observation.
Add a machine at the front of the store selling non-refundable, refillable, good forever store debit cards in exact requested amounts. Machine takes cash and cards, gives change on cash if necessary, and accepts no bills above $50.
Very small investment for increase of customer base, including the Amazon-less and the internet-less with the bank-less, and allowing for anonymous purchases for those not liking to be tracked.
Customer goodwill would increase, also earned publicity. And some cards would not be completely used, compensating for the different but increased information, mostly from new and recovered customers, proverbially the hardest gains in retail.