So my observation is that a blacker background on a tv does not make the programming better or worse. However if you focus on the blackness of the background you can make yourself unhappy with your equipment. Is that a smart thing to do?....Is there such a thing as 'optimal stopping' in audio? I think so.
I would agree that there is such a thing as optimal stopping. What I have found recently -- by adding a relatively inexpensive linear power supply to my streamer -- is that the content (or "programming") *did* get better because I was able to hear instruments and notes that were not previously available to me. If that occurred by adding better power to my whole system, it would be quite reasonable to do.
So, perhaps, one metric to answer your excellent question -- roughly, "When is it optimal to stop improving the system?" -- is this: *when no more content is added or improved significantly.* Or even, *when I cannot justify spending this money, here.*
Knowing when to stop is kind of a judgment call which comes down to each of us. It's that same part of us that knows when to stop eating at an "all you can eat" buffet!