What is your experience with the law of diminishing returns ?

A different perspective...
Twenty years ago, when I had better ears and a less expensive audio system, I made a visit to the local high-end store. I had $5k invested in used equipment (ARC tubes, Soundlab Dynastats, HGS sub). Listened to my reference music on an $80k new system (Martin Login top of the line bi-amped with four Krell monster mono amps, etc.).

The higher end system definitely sounded a lot better. The 3D soundstage really stuck in my mind. After an hour of listening I walked out of the store with a big grin on my face thinking I didn't really hear $75k of difference.

Yeah, we can try to fool ourselves and at times we can even be successful.

Cables and power cords are components and as such they can sometimes give as much sound change as active components.
However, speakers might not be good enough, so whatever you replace in the chain give about the same magnitude of change.
Or, your system is a total mess, completely in disarray, and whatever you do it won't matter much - same mess.

I think that Elizabeth has it right in terms of marginal cost / marginal gain curve.   One might quibble about price point (not a lot).

Then the owner’s financial condition as well as sensitivity to the curve must be factored in.

For myself, I lust after the new 30.7 Maggie’s, but a house downsizing leaves me a room 23 x14, not quite large enough. At the end of the day, I love where I am I terms of my system, but we always see the greener grass smiling at us, LOL.

Also, often lost are the relatively inexpensive tweaks, I.e. room treatments, etc.