Burn-in and Equipmemt Value

THe premise of the op has a flaw. The problem is the fact only a small percentage of people care about burn-in as a major issue.
Even though a lot more folks think burn-in exists, they are not very concerned about it.
So it never makes it as a big deal in buying considerations... (except for perhaps a very few people.)
So the numbers of individuals are just not big enough in the overall scheme of things to make 'used' a better proposition.
Most of the time the main consideration in buying 'used' is just to get something cheaper. And 'cheaper' is the operative word. The general attitude is half price is the point for most used stuff. Only particular 'famous' equipment can get more.. And plenty of older used is well below half price.
Some of my 'happiest' used purchases were around 25%/30% of list. And I would never have bought them at 60%.. So....

But then for a few items I really wanted, and were rare I did pay over 60% of new.. So it all depends on the market, what one wants, availability...

Burn-in is free. It doesn't cost anything. Why would anyone pay more for something they can do for free?

If the "only" way to break-in a component was to pay to have it done, then maybe it's worth more, but it isn't.

Some buyers might even look at it as being more used and want to pay less for it...

Burn-in is free, bit does take time. A long burn-in takes some effort.

Speakers are mechanical with physical movement so it makes perfect sense that things can change with use. I am not sure how much of what I hear with my tube amp is just warm up vs. actual break-in, but it seems that something has changed. I don't believe that solid state really benefits from warm up or break-in and this goes for cables as well.

People pay for cable burn-in so it seems that those same people might pay more for a demo amplifier or speaker. The used market come with more risk of abuse so I can understand paying less.

Why is it that burn-in is generally assumed to always result in a positive change? Why can't speaker cables wear out?

A cable can have oxidation/tarnishing issues and mechanical joints can weaken from repeated handling, but what exactly in a cable can wear out?

"Why is it that burn-in is generally assumed to always result in a positive change?"

The designer will judge the finished, or pre production product, only after its broken in. So if you want to get best results, at least from the maker's perspective, you need to listen to a unit that is broken in because that's how they listen to gear during the design process.