You can open a terminal and come back in a month and it will still be there. Unlikely kunernetes where containers regularly go down.
You can obviously achieve high reliability with Kunernetes, but you'll need queues and retry logic, that you don't need with SystemZ.
You mixing apples and oranges there, by comparing kubernetes workloads to mainframes. Kubernetes isn't really designed to serve long-persistent workloads of that fashion. Although tbf I've had VM's that last for years and real hardware (albeit Sun) that's had over a decade of uptime, so I'm not sure what all the fluff is about.
I assume the achievement part is that any CPU can be replaced transparently to the terminal running?
Not a big deal today, but in the 80s hard drives died more frequently and a bank losing just 5 seconds of transactions could cost them millions.
It wasn't cheap, but it made a typical "modern" high availability setup look like a crude homemade toy.
But as impressive as it was, there are just very few places where that impressiveness provides enough value to justify the cost. And having to deal with IBM.