Who uses long interconnect like in dealers?

I do. Two 6 meter pairs from preamp to amp. Good question (your 2nd to last one). I have one 6 meter pair of Discovery Signature IC if you are interested.

My source equipment is on a side wall and my amps are between the speakers. My IC's to the amps are 4m and my speaker cables are 1m and 1.25m (system is bi-amped).

I have also noticed that there are very few long IC's on the used market. Maybe people don't do this much because the initial cost is so high. Good IC's can cost $1K or more, and I'm not talking about the really expensive stuff. I would love to buy some Audience or AZ 4m cable but just can't quite get there with the finances.

So I'll just continue to watch for that rare used pair to come along and hope someone else doesn't beat me to them.

Good listening.

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.