First, you need more POS stations, because customers are not as fast as trained/experienced cashiers/baggers. So you need more machines to serve the same volume. That cost might be worth the lower labor costs.
But those extra stations also consume space. Picture a Walmart that dedicates one entire edge of each store to a row of checkout lines. It's easy to imagine that 10% of their public floor space is devoted to that function. If it takes 50% longer to self-checkout, you might need 50% more floor space to handle the same volume of purchases. That's not completely accurate because stores don't convert all their checkout lines. But you get the point.
I have noticed that most places compensate for the space requirements by drastically compressing the space for a station. No more conveyor belts. No more funneling customers throught a gauntlet of magazines and chewing gum. They might replace 4 checkout registers with 10 self serve stations. In many ways this can be better for the customer but worse for the business. There's a lot of money to be made in all that merchandise immediately surrounding checkout stands. Even the endcaps on the aisles closest to the checkout lines are worth special attention for maximizing sales. Losing this hurts the business.
I'd be curious to see the results of rigorous studies. In the meantime, as a customer, I like the self checkout stations.
I'd be super curious to see things completely flipped on their head. I'd like to see a grocery store where you walk through aisles that have only one item per SKU on the shelves and you point your phone at the qr code to "buy" one. The store app lets you indicate how many of that item you want. In the back room you have robots feeding your items up to a boxing station. In the time it takes you to walk to your car after pushing "Finalize and Pay" your order is waiting at the will call loading dock as you drive through.
No shoplifting losses, dramatically lower security costs, better and cheaper inventory management... A long list of benefits. And of course an offsetting list of increased costs. For example: smaller sales floor requirements, huge loading dock requirements.
