The successor IBM Mainframes are still alive... for the time being.
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they also moved on three more CPU generations since that redbook, to z17.
I think it's Linux on Z that makes it sexy and keeps it young, in addition to a number of crazy features, like a hypervisor that can share CPUs between tenants, and a hardware that support live migration of running processes between sites (via fibre optic interconnect) and the option to hot swap any parts on a running machine.
It's doing a number of things in hardware and hypervisor that need lots of brain power to emulate on commodity hardware.
_and_ it's designed for throughput, from grounds up.
Depending on your workload there may be very good economical reasons to consider a mainframe instead of a number of rack-frames.
you don't have racks with blades but one box or two, that you upgrade every few years and other than that your hardware side is covered.
rewiring networks, reattaching storage etc all remotely, CLI, and most importantly you only pay what you use.
you need one guy or a consultancy for the initial etc up and for fancy Config changes but once it's essentially set up you simply have a large Linux data centre and do Linux stuff.
oh and you can hot plug additional physical CPUs into a running VM and pull it out and reassign when it's not needed, likewise RAM. that's somewhat difficult with commodity hardware.
and there _are_ companies that value this and choose Linux on Z as a pure Linux deployment.
but you have a point. if nobody in the building knows this, nobody's gonna call IBM and ask for a quote. it's simply not on anybody's radar.