On your second to last question, perhaps the answer is it's too expensive to upgrade? I use a 40 foot run between my preamp and power amps/active crossover, and that has always been the last thing that would get upgraded because of the cost. I've tried to make that as good a cable as I could, given the limited choices available for that long a run, to avoid having to change it. And it's also possible, I suppose, to send the long run to the manufacturer to reterminate in shorter runs, then sell those, which would be an easier sell in the used market.
Who uses long interconnect like in dealers?
Ideal setup has front end components close to listening chair and run long interconnect between pre and power where power amp sits in between speakers; I have seen many high end dealers to adopt this setup and result is usually more than great. This way front end components do not obstruct speaker imaging and can also avoid bass heavy (vibration) location. But if that is ideal, why do I rarely see long interconnect for sale in used market? I don't think people only upgrade components and short interconnects?