Little's law doesn't imply what you said, either. It tells us the number of people in the system based on arrival and wait, but nothing about throughput.
And you're right about throughput, I was implicitly holding the number of people in the queue constant, and you can obviously add arbitrary delays and increase the number of people in queue without touching throughput.
Just like the website load times, you should be focused on the 99th percentile and the median. Single line reduces those metrics.
Really this boils down to whether the business is interested in saving a bit of money (fewer cashiers per purchase) or improving customer experience.
The metric you want is to figure out what wait time constitutes good/acceptable/bad service, and what percentage of people get that service.
As a customer, all you care about is latency: How long you spend waiting in line. As a store owner, you also care about throughput: How many cashiers you need to process some number of customers per unit time. A store owner that cares about customer satisfaction will still care about latency, of course.