z is up to the minute in terms of clock speed, CPU process and such. They have a RAIM system which is like a RAID array for RAM which must cost something in latency but lets you replace failing RAM sticks when the machine is running. The rest of the industry will have something like that soon based on CXL.
Being up to the minute in terms of clock speed doesn't matter that much when, for example, DB2 on the AS/400/iSystem has a much higher latency and handle much less requests per second than an equivalent Xeon.
Replacing failing RAM sticks live are also something that, while quite cool and impressive, it happens to be quite pointless in today's world of clusters.
Current AS/400 is almost retrofuturistic in both its cool factor and disconnection from current actual needs.
This is one of the mainframe's key selling points. Not that I am implying z/VM is user friendly.
But instead of managing a fleet of x86 servers, switches, routers, storage boxes (you may need a few of those), all liable to fail without warning, you have an exquisitely built datacenter that fits in half-a-dozen 19" racks (counting 2 for storage) where redundancy, failover, and easy maintenance are built-in at the hardware level.
