Its weird to have companies say they're moving from Mainframes to the Cloud. The Mainframe is a sort of cloud, each one has usually double the spare capacity and you call IBM to unlock it. The cost of running a Mainframe is measured in terms of MIPS based on amount of compute used and there's compute reserved for offloading things like JITs (zAAPS). Essentially a version of cloud compute costs. I can understand if yo…
Here's the thing: A mainframe may have spare capacity, but you still have to call IBM to unlock it. In order to make a Z run in a cost-effective manner, you need to run at 90%+ utilization at all times - which is excellent for batch jobs that can be scheduled, but is difficult to achieve with on-demand loads.
You're paying based on compute available, not just compute used (unless I'm misremembering our contracts back in the day). Sure, the amount of available capacity can be changed, but a phone call to big blue is not automated. Cloud autoscaling is.
You're right that IBM mainframes are far more modern and cost effective than we assume. Sadly, they are still behind the curve of cloud hosting.