This is the reason. IBM Mainframe business grew 60%. The modern mainframe is the best state of the art platform for computing, in both reliability and efficiency.
Also, IBM mainframes are wonderfully isolated from physical hardware. They could change processors in the next model, and users would notice a small delay as binaries were recompiled on-the-fly the first time it was used.
They surely could extract more performance from the hardware by shedding layers, but prioritized stability and compatibility.
> Also, IBM mainframes are wonderfully isolated from physical hardware. They could change processors in the next model, and users would notice a small delay as binaries were recompiled on-the-fly the first time it was used.
This was with AS/400's move from their own CISC processors to POWER. While you could pull that off with mainframes, it'd be recompiling actual native binary code. IBM mainframe architecture is very well defined and documented (sadly, unlike AS/400).
At this point in time it can have more cores and more memory than a Z, and likely higher performance in benchmarks, but the architecture is closer to a minicomputer than a mainframe.
They surely could extract more performance from the hardware by shedding layers, but prioritized stability and compatibility.