So, you buy that, world loses a Terra Cresta. Go to deconvert it, somebody somewhere else torches an unconverted Mappy to feed the need for parts. And don't think that isn't exactly what happens.
Why do you assume cabs are torched to 'feed the need' for parts? Here's an alternate scenario... a Mappy has been sitting in a barn or warehouse rotting and is beyond repair but the components are all recoverable and for sale. A buyer purchases them and puts them back in a dedicated cab like this. I don't profess to be as you knowledgable as you but I don't get what you find rare about a conversion like this, it seems easy to replicate in any cab.
Certain parts tend to pop up loose and survive conversions. Boards and marquees particularly. Bezels a little less commonly since they were often scraped to be reused for the conversion.
A loose control panel usually means the game was destroyed. Might have been last week, might have been 20 years ago, although it is more likely to be within recent history to have been stored without being junked for the whole 15 years or so where it was worthless and useless.
Wiring harness, that pretty much means the remains of the cabinet are still smoking. I have never found a wiring harness in any raid that wasn't a new unused Jamma harness.
Here is a fun game you can play, next time a Wells gardner color vector monitor pops up on ebay check the seller's other items, you are almost certainly going to find a Space Duel wiring harness at the bare minimum, usually everything off the machine is listed.
Ultimately the issue is that most kit-only games are simply going extinct. Every time a collector gets one he feels the need to deconvert it (maybe another one gets torched, maybe it doesn't, I know I would be lighting paperboy project cabinets on fire myself with the prices the parts for that game have reached). Every time a money guy gets a conversion game he sells the board to a board collector (most of these boards then vanish from circulation and never hit the market again), and then slaps in a 60 in 1 boardset and sells it to some chump.