I recall reading that when they introduced self-service checkouts theft went up significantly; people were much less reluctant to still if they were only being served by a machine. However the solution was not to simply go back to use manned registers again, but instead make the systems for theft detection more sophisticated. I wouldn't even be surprised if the final systems that we have now are not even cheaper than manned registers, but the way the transition worked was that it was always sold on "potential savings" on idealised conditions.
