And who repairs the robot?
My office in Seattle has an Amazon Market across the street. It's true that there are no cashiers, but there are two people at the entrance to make sure everyone follows the rules; that they don't move a sandwich item from one bin to another; that must stock everything strictly so that The Machine can recognize it, and so on. This is in addition to whatever engineering resources are needed to solve problems; fix electronics when something happens (hey, the soda exploded!).
So, yeah, no cashiers. Good job.