@russhealy,
'Perhaps this is a bit like Frances Fukuyama claiming history ended when the Cold War ended, ensuring liberal democracy a long history devoid of any alternate system of governance.'
Yes it does look as if there's a convergence of a science based opinion of what constitutes a good sound lead by the likes of Harman Kardon, Audioholics and Audio Science Review.
Maverick designs such as the Ohm Walsh's and Tekton Moab's do seem to be getting thin on the ground.
The question of when to stop upgrading must therefore be an individual one dictated to by the majority of one's own listening material.
[The alternative would be let your choice of music be driven by your system, but that can't ultimately lead anywhere good other than visions of individuals with $$$$$ systems playing the same 5/6 fantastically recorded discs over and over again...]
If you've already passed that unfortunate point of too much resolution (you'll know by the range of your listening material and level of satisfaction) you might have to consider downgrading to something more suitable.
Many already have to their satisfaction.
Another option might be to push for an increase in industry recording standards.
Unfortunately according to Steve Guttenberg and others with experience, the industry does not see audiophiles, who constitute less than 1% of their business, with any type of interest.
They know we'll buy the music anyway, so they're far more likely to focus on those sectors that don't.
For those remaining audiophiles who still seek the maximum sonic pleasure from their music there is the additional, albeit time consuming option of searching for the best reissues/ remasterings/ pressings etc.
Sites like Steve Hoffman's Music Corner have been devoted to doing exactly this for quite some time now.
https://forums.stevehoffman.tv/forums/music-corner.2/