Why Don't They
..use/adapt QEMU and emulate S/390, x86, SPARC, PA-RISC, Itanic and all that on POWER. Emulation/Translation now approaches Native Speed.
The S/390 CPUs are artificially overpriced to pay for total system R&D costs anyway. They are a kind of dongle for the mainframe software. Just adapt the POWER/QEMU pricing so that it results in a competitive MIPS/$ rate.
IBM could then say
"We run ANY operating system (zOS, AIX, OS/400, VM, Windows Server, Linux, Solaris, HPUX, BSD, MacOS X*, Haiku,...) on our hardware."
"You can use each and every processor for any operating system and workload".
The current solution is the typical kind of hodgepodge which only The Pointy-Haired invent.
*at least technically. Legally they would have to talk to His Royal Highness, The Steve of California before doing that.