Software licensing is expensive on them, but the people to keep your 99.999% uptime on your Kubernetes cluster aren't cheap either.
And software licenses are one of the reasons why LinuxONE machines exist - they don't run z/OS, so you don't pay those licenses. You can even start a dozen VMs under an LPAR and run your Kubernetes cluster as if it were running on more common hardware that just never, ever fails. IIRC, you can run a special version of z/VM to manage your Linux VMs if you don't want to run Linux on the LPAR and use KVM for your VMs.
And software licenses are one of the reasons why LinuxONE machines exist - they don't run z/OS, so you don't pay those licenses. You can even start a dozen VMs under an LPAR and run your Kubernetes cluster as if it were running on more common hardware that just never, ever fails. IIRC, you can run a special version of z/VM to manage your Linux VMs if you don't want to run Linux on the LPAR and use KVM for your VMs.