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.