"Touchscreen, LAN, WiFi, USB, you name it, can absolutely be done, and for little effort on the Keysight side"
I will dispute the "for little effort" part of that claim.
As much as we drool over the gorgeous hardware in the 3458A, more than half the magic is in the software.
While there are things you can do to improve that firmware, adding "Touchscreen, LAN, WiFi, USB" with "little effort" is not one of them.
Just moving the firmware to a CPU which does not execute mc68000 instructions would be a major undertaking, due to the way command-dispatch is implemented.
Ensuring that the new firmware measures *exactly* the same way as the current firmware would take a lot of effort over a couple of years.
Of course one could design a UX unit with a touch-screen, which pretends to be the existing screen & keyboard electrically, but it would be intolerably slow.
Likewise, one could stick a small embedded computer on the GPIB to offer USB/LAN/WIFI interfaces, but that would increase the command latency.
I'm pretty sure that if you poll the people who pay list-price for new 3458A's, they will either tell you that they press a few buttons to do ACAL and to select what to measure and have nothing hooked up to GPIB, or they have something hooked up to GPIB which they *really* do not want to have to change in any way, because it is part of the certification of their manufacturing line.
In other words: Yes, everybody would absolutely expect better UX and interfaces on a new design, but to the paying 3458A customers adding that would be a good reason to look for a different solution instead.