I agree with you there. Most mainframe tasks today aren't that heavy on the compute - most relied on the amazing I/O stack, and that makes it easier to replace

I agree with you there. Most mainframe tasks today aren't that heavy on the compute - most relied on the amazing I/O stack, and that makes it easier to replace them. Heavily vectorised code with modern instruction sets can also be more performant than a lot of the older compute chips, but that requires more rewriting of code, and someone who intricately understands both the code and the math. Which makes modernising much more expensive. The VM approach is a simpler way to get you most of the way there, but replacing heavy compute stacks is usually going to require a decent bit of investment.