Of course, all of these companies also have x86 servers that run the customer facing web apps and stuff. You really wouldn't want to waste your MSUs running web servers, although I'll admit I don't know if these new Linux mainframes are charged by rolling avg MSUs or other MSU metrics. Traditionally, Linux on Z has had more favorable pricing due to the ability to use IFLs and basically just declare that a cores MSU usage doesn't count towards your licensing since it's been locked to only running Linux.
Then you have half a mainframe of capacity lying about, and why not run some linux workloads on it.
I find it _really_ hard to believe that outside of a few really narrow niches anyone is really buying mainframes to run linux workloads. Its just not cost effective.
PS: and then of course IBM gets to add it to their total "sold capacity" for the quarter to make it look like the business is growing. So they _want_ you to use all that idle hardware.
AIUI, Oxide Computer is targeting the "private cloud" space which is sort of adjacent to this stuff and could easily intrude on it. Of course they're based on AMD EPYC hardware, but the rest of the system is custom designed for ease of management and reliability.
The thing that recent grads(weather engineering or business) tend to miss here is that for a lot of those organisation the central mainframe system and the organization's policies and culture is pretty entangled to the point where you cannot just replace either.
[1] on time, on budget and without leaving some aspects of the old system running under some sort of virtualization.