It's hard to define the point of diminishing returns. It depends on tastes, how rich you are and what your listening space is like. In a flimsily built wood frame apartment building one hits that point fast because high dynamic range and bass extension, the two things which are costly, are lease breakers.
We all know there's lots of expensive stuff out there which is total BS.
Perhaps for people with means, no neighbor problems and enough space the $22,000 Salon 2 represents the top. Beyond that it's just pissing money away.
Everyone has quoted a different dollar figure for the point of diminishing returns, depending on tastes, circumstances, wealth, etc, etc.
But the common ground is that there IS a point of diminishing returns, even if it differs from person to person.