This mainframe system from the 1990s was so much simpler
https://www.ibm.com/docs/en/cics-ts/6.x?topic=sysplex-parall...
even if it wasn't as scalable as Kube. One the other hand, a cluster of 32 CMOS mainframe could handle any commercial computing job that people were doing in the 1990s.
Seems the causality is going the wrong direction there. Commercial jobs were limited by mainframe constraints, so that's where job sizes topped out.
It's not simple but it's not opaque.