Respectfully, the hell you say. Cloud 'autoscaling' is something that has to be tenderly maintained by software engineers, who expect to be paid salaries and benefits and so on. It's not like FedEx can just rsync their data into the cloud and have all their software run forever. Instead, they need the engineering team that manages their existing data workflows, and now they also need cloud engineers to translate that into something that won't bankrupt the company, since they're moving from the mainframe world (where you keep your system loaded to an efficient price point) to the cloud world (where you are billed by the second).
Moving your compute spend from capex to opex is a perfectly valid move, but pretending the reason is that someone has to make a phone call once in a while is kind of bizarre, when the alternative is having to hire a whole new cadre of techincal talent.
I agree the occasional phone call removed from your workflow isn't reason enough to justify a giant cloud migration... but it is one of many reasons.
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.