I paid $5K for my Pass Labs X250.5. 15 years later, should be able to get at least $3K. Paid $6K for my CJ ET5 pre ten years ago. Again, I think it will sell for $3K+. Maybe I am lucky. Who knows?
Having said that and IMHO, there are other reasons, not yet mentioned, for the typical "dollar value drop" phenomena.
One example is certain manufacturers that offer "new and improved" products on a two to three year cycle with no upgrade path to the owner of the previous unit. Another is manufacturer advertising, audio reviews and those who sell the gear. And then there is the "must have the latest, greatest" mentally driven attitude amongst some consumers. And finally, the exhorbitant price structure that is so often prevalent in hi end gear. I am not saying all of the above are applicable to all parties in the hi end audio universe or that new tech and tricle down improvements do not exist. But I often wonder and speculate what fuels this "market reality".
The new models are usually identified by numeric or alphabetic iterations and typically result in a signicant depreciated price for the previous model in the used marketplace. I personally believe this is, in part, intentionally driven to maximize corporate profitability. And by purchasing the latest model, consumers have "enabled" this cycle to continue.
To me, the solution is intuitive and obvious but like many other products made, most audio gear has sadly become just another "disposable" item. Sometimes, less is more. Best.