How do you imagine cash registers work these days? With 5,000 stores worldwide, 40k servers is 8 computers per store, which doesn't seem excessive to me.
I worked with a Danish retailer with +3000 store in ~50 countries, and even adding their webshop on top and they were closer to 200 (maybe 300) servers (most VMs). Then you need the ActiveDirectory, office IT, all that stuff, with redundancy and it adds up quickly... but not to 40K.
What I will say that people forget is that production might be 8 beefy VMs, you still need to replicate that to a number of test systems, staging environments and so on. So a 8 node production cluster because maybe 24 servers when accounting those other environments.
I can see how you hit 40k pretty quickly.
and dont forget that different countries have different rules about customer private data and payment information. if you send eu customer info into the uk for processing you might be breaking privacy laws. some places do automatic tax reporting where you need to send info to a country specific tax office api, get back a code and print it on the receipt.
cash registers dont work alone. its connected with inventory management, employee perf monitoring, payment processing and other things that make more sense as a store local service sending regular reports to corporate instead of waiting for round trips on every operation.