Technically that would mostly work but IT leaders at banks and insurance companies would rather pay high premiums to IBM then stick their neck out endorsing anything risky. Renegotiating with IBM is the safest career move at such places. Cloud is steadily edging out mainframes at banks but a lot of old guard IT leads are circling their wagons around old blue because learning how to do high resiliency on cloud is new and scary, but old blue is safe and well understood.

Using safe and well understood tech to handle my money sounds like a good thing.

It's hard to understate the value of safe and well understood for core systems. After all it's not like a massive migration to a new technology stack has ever blown up in anyone's face, right? (</s> if not obvious)

>then why not just run the workload in a z/OS emulator on commodity hardware?

Who are the vendors that provide a migration path from mainframe to a set of commodity hardware running an emulator, and subsequently provide continual maintenance and support?

The big cloud vendors have partners who specialize in such work. Random one from the first page of Google results: https://aws.amazon.com/blogs/apn/migrating-a-mainframe-to-aw...

part of this is the 'partnership' arrangement.

Global 100 company gets IT from other global 100 company

vs

Global 100 company gets IT from small mainframe support shop

there's a question of shareholder liability, being able to adequately sue them for M's of $, expectation they will be around in 20 years, etc.

Total non starter. IBM won’t license the software for use on an emulator.