You know, that analogy makes me wonder if there's scope for a similarly tiered approach in supermarkets, like hotel rooms or better, airline classes... is there scope for a "business" or "first" service in the better supermarkets like Waitrose or M&S?
A special set of checkout aisles with extra staffing that you can use if you're prepared to pay an extra, say, 3-5% on your shop. The exact percentage would need a sensitivity analysis to determine, but you get the idea. Charge a bit extra in return for a smoother shopping experience.
I wonder if any of the big players have done any market research on this concept to see if there's a large enough small pool of customers willing to tolerate this charge. Probably not, admittedly, but I'd love to see the numbers.