IMHO the law of diminishing returns applies to those who have limited financial resources to spend on audio gear, while still trying to get the best sound that they can for their audio dollar.
For those who have what is essentially endless financial resources, they can afford whatever audio gear they desire, while enjoying this hobby at a level that most of us can only dream about.
As always, this is just my subjective opinion and should thus be taken with a grain of sand. 😊
The principle (not a law) of diminishing returns apply to everyone at any price level why ?
Because this principle emerge from the distance separating subjective and objective acoustical gear evaluation in a room by some specific ears and the objective/subjective qualitative evaluation of the material gear design level in some acoustical and system context...
Then it is preposterous to describe this principle as a fate pertaining to low budget perspective versus rich audiophiles...
The principle apply to all for any system/room...Ignorance of it come with a price to pay...
For sure this principle as a meter and as a measure is relative then not absolute...But it exist for all ...