Comment by froh

what you hint at is that most workloads today don't need most of the mainframe features any more, any you can move them to commodity hardware.

There is much less need for most business functions to sit on a mainframe.

However the mainframe offers some availability features in hardware and z/VM, which you need to compensate for in software and system architecture, if failure is not an option, business-wise.

and if your organisation can build such a fail-operational system and software solution, then there is no reason today to stay on the mainframe. it's indeed more a convenience these days than anything else.