But now we have the 21st-century mainframe we like to call the cloud, where everything is shared. So I upload a container image with the vampire vuln intending to read all the activity on the host. Other customers jobs, the OS itself, even steal internal keys used at Amazon.
The motivation to do this kind of attack now is incalculable.
There is the "just use OS/2" strategy of the 1990s (referring to the period from 1995-2002 where you could sit happy in your niche OS that researchers and BHs were not targeting): in this case s390x probably has the same problem but no one is paying attention to it (other than a 3 letter agency, of course), so the chances of a payload reaching that arch are very close to zero.