The scary fact is that some audio equipment does indeed benefit. I blame it on "switched mode power supplies" in many common household items from battery chargers to computers - these can be noisy. In the past everything used to be powered with large isolation transformers but not today. I have one component that benefits and it is digital - I don't know if it was jitter or what was wrong but a power conditioner helped. I tested two conditioners and both had the same effect on only one component. Naturally I will change this component next as I suspect less than adequate power supply isolation to the electronics somewhere.
I think you are right in principle as the best designs with overbuilt good power supplies should not need power conditioning in most normal situations.