Diminishing returns is a marketing/sales pitch created by the audio industry to keep budget conscious Audiophiles content ...
That is simply not true. The law of diminishing returns is a well known economic principle, also known as the law of diminishing marginal returns.
I do agree with @ghdprentice that it's really not part of the equation for those into high end audio. We're already well into that curve because "reward function is not linear at all."