I think the problem here is that nothing at a higher layer is mitigating the effect of the attack. It's not so much a choice to put the complexity where it is c

I think the problem here is that nothing at a higher layer is mitigating the effect of the attack. It's not so much a choice to put the complexity where it is cheapest as it is a total ball-drop on overall composite system correctness (and thus security). From a different angle: I think your point is fair but I also think that for it to apply to this situation, the memory vendors would have needed to loudly and openly say that they were invoking that tradeoff so the OS vendors could adjust. Presumably that would also result in a lot of benchmarking being done to see if the net effect of a physical-layer vulnerability and a software-layer mitigation was actually a net positive.