I think the #1 use case here is allowing AI/cloud workloads the ability to execute against the mainframe's data without ever leaving the secure bubble. I.e., bring the applications to the data rather than the data to the applications.

IBM could put an entire 1k core ARM mini-cloud inside a Z series configuration and it could easily be missed upon visual inspection. Imagine being able to run banking apps with direct synchronous SQL access to core and callbacks for things like real-time fraud detection. Today, you'd have to do this with networked access into another machine or a partner's cloud which kills a lot of use cases.

If I were IBM, I would set up some kind of platform/framework/marketplace where B2B vendors publish ARM-based apps that can run on Z. Apple has already demonstrated that we can make this sort of thing work quite well with regard to security and how locked down everything can be.