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Groceries are a highly competitive market so a large fraction of cost savings end up being returned to the customer in lower prices.It doesn't look this way from customer perspective, so I propose alternative interpretation: it contributes to hiding inflation, because instead of seeing the prices rapidly rise, we see them rise slower (further slowed down by "shrinkflation" and related shenanigans), but the quality of service goes down too. So what we get is ever worse shopping experience and lower inflation rates reported by economists.
(Not to mention, shopping taking more time because of this "optimization" is effectively even more hidden inflation.)
> From my perspective self checkout is a latency optimization. The person in front of you in line may have 3 items and still take an unreasonably long time finishing the transaction, but replace one line with 4 machines and the line keeps moving.
It's a good reason to have both. Because in practice, 2 out of 4 machines are, at any given moment, locked and waiting for a supervisor - and many stores "optimize" further, by eschewing a dedicated role and just tacking responsibilities on to existing work roster. Meaning, the supervisor you're waiting for is likely staffing a checkout point or unloading boxes in the back of the store at the same time, so you better get ready to wait.
Ideally, the store would have both automated and staffed checkout, and direct those with few discrete items to the former, where a dedicated supervisor would ensure problems get resolved quickly. That would indeed optimize latency/maximize throughput.