Getting (and maintaining) a local mainframe in the first place is also not cheap though. I won't hazard a guess to which one might be easier to keep up and runn

Getting (and maintaining) a local mainframe in the first place is also not cheap though. I won't hazard a guess to which one might be easier to keep up and running, but at least in my bubble it would be 100x easier to hire more k8s admins than to hire qualified mainframe admins.