I think it is a fair point to acknowledge that things like cables and wire fall under the "law of diminishing returns"... I DO believe that there is something positive to be gained from almost any decent system by upgrading past the 'freebie' RCA interconnects that come with some equipment and the 'discount' bulk speaker wire towards products that are at least somewhat higher quality. I myself swapped out those cheapo free interconnects with some modestly priced (e.g., $20 each range) interconnects and absolutely noticed a reduced noise floor and especially a change in the signal from my turntable. I also upgraded speaker cabling from bulk 16 gauge to some SVS shielded 14 gauge and again, most notable difference was less background noise during quiet music passages, etc.
I also recognize that there is the POSSIBILITY of greater sonic differences / performance when utilizing equipment that is more discerning (read that as "higher end") than the equipment I have. I also believe that it is entirely possible that noted differences above a certain threshold may be entirely listener bias and not anything measurably superior.
Lastly, I would add that any hearable difference likely decreases in both scope and range as you move up the cost ladder... and you likely will end up chasing truly almost intangible improvement with increasingly larger dollar purchases...