My understanding that this clustering is more to handle external factors. It allows mainframes to be placed in different buildings anywhere from the same campus to up to 200kms away ideally providing enough distance to deal with natural disasters.

Mainframe itself (the "fridge") is a distributed system, as well as human brain is. It's all the management tooling that makes it appear as a single system.

One possible reason why: customers might want to be able to upgrade their hardware/OS/whatever and retain their 5 9s.

Good luck trying to get five 9s when you're limited to a single DC. Cross-DC SYSPLEXing (GDPS) got us to five 9s and beyond without breaking a sweat. Our systems can never go down, ever.

From VISA with love.

Still remember those days that these are for those transaction processor marketing against mainframe we used. Not in the field. Where they go to ?