Modularity could work on mainframes too but it mostly didn’t. Mainframe cost per transaction can be very low, and yet here we are. Tech at scale always passes through a cheap/worse is better phase.

Take a look at the open compute project for example. Instead of having power supplies in each server, they have a lesser amount of large ones for example.

Economies of scale wins in big compute projects too

Which is why comparing mainframes and nuclear reactors is a bad analogy. A better one would be datacenters vs individual PCs. If the goal is efficiency, datacenters win everytime.