Mainframe? Are you sure?
To me, probably being of a certain age, the key distinction has always been: how much data and in parallel, can one shove through the computer at a time. "Lots continuously" = "mainframe", "quite a bit, but rather bursty" = "mini computer", "user interaction limited" = "PC".
So the designation "mainframe" has always hinged on the number and bandwidth of connections between the CPU and the storage devices and other streaming peripherals. While processors have got faster, I don't think either the interconnects or the hardware that handles them inside the "mainframe" have kept up to the same extent. Indeed the tendency to land stuff on buses, as opposed to direct connects closer to the CPU(s), will always add contention and slow dataflows down.
A "mainframe", then, is optimised to get the largest number of "streams" of data into, munged a bit, and then out of the unit as possible. Just because CPUs, discs and their interfaces are faster, doesn't alter that basic distinction.
I wonder just how far IBM can go before someone notices that they are just buying a (very) large and expensive mini?