Yes, it’s antiquated because we used to be factory workers, where we’d just have a start-stop time because we weren’t automatons that could work anytime. The factory had a beat because product Z takes N steps.
But a cashier is basically in a sales factory; they mediate sales between shoppers and the department store. So, your “antiquated” is just a privileged perspective.
Yes, basically all of these missives and thought experiments implicitly exclude retail while studiously avoiding admitting that this is the case. Retail stores need to be open a sufficient number of hours at the right time(s) of day to capture their potential customers. Unless you’re not able to handle your checkout line or your restocks for whatever reason, “efficiency” doesn’t really enter into it. Not in the sense of employee time, anyway. Maybe the world’s best 7-11 clerk could check out a thousand customers in an hour but just because he can that doesn’t mean it makes sense to close the store at 4:00 PM or only keep it open three days out of the week.