I don't understand the argument around microservices purporting to fix scalability. Say your system has modules A, B and C in one app, and that A requires more

I don't understand the argument around microservices purporting to fix scalability. Say your system has modules A, B and C in one app, and that A requires more resources while B and C are relatively unused. Won't B and C just run less and thereby use appropriately fewer resources from within the same system as A? Are microservices just an aesthetic separation (it feels nice to know you're "only" scaling up A)?