there are both categories.
Some things are marginally better and incredibly expensive.
Other things, such as garden hose power snakes and these "audiophile crystals" are just pure scam.

I'd be all up for some diminishing returns but the price premium is nowhere close to adequate. For the price of an overbuilt headphone amp you can buy a soldering iron and parts for three amps that are four times as overbuilt. And include a metrology-grade DAC in all of them.

And, yes, my headphone cable is oxygen-free copper. I simply chose the cheapest suitable cable I could find at Thomann, the stuff is so cheap they're throwing it in there for diminishing returns in sales.