The legit use case for companies like Google/Amazon etc is only to sell it to customers. None of these companies use K8s internally for real critical workloads.
Ehm, that is simply not true. Google built it for themselves first. It is essentially the open source version of the internal architecture. It gets used.
I worked at google. k8s does not really look at all like what they used internally when I was there, aside from sharing some similar looking building blocks.