Since I had the time, I read through this whole thing. Interesting, but not surprising, to see nothing has changed since it was started 6 years ago.
It is my belief that the 10 yr rule thing shouldn't really apply to the 1800, at this point in time. Nothing is different underneath the cosmetic changes to say "sorry, we cant work on your 2002 Wing". To use the excuse of hard to find parts is pure BS. This generation of Goldwing is now in its 16th year. Long in the tooth for sure, but that also makes it easier to source/stock/sell parts.
Motorcycles are certainly a different market than cars. It is much smaller. It's seasonal in most of the US. It does not benefit from mass quantities. I too am completely dumbfounded by the whole refusal to work on them. The ones that do, benefit greatly due to being one of the few "go to" places. Auto dealerships have proven that service departments are a very lucrative source of revenue. The same principals apply to managing a successful motorcycle facility. The downside is that it takes a substantial investment to get a decent ROI. Therein lies the problem. Many people do not want to pay a shop what it takes for them to be profitable and successful. We all love to have a "go to guy". But they usually are 1 man shows with no succession plans to keep it going. When that guy is gone, everything is gone.
So the real problem, IMO, is the potential cost of repairs to rise beyond estimates due to unforeseen circumstances from the vehicles age. If people are made aware of this UP FRONT, there is usually no problem. Most people are understanding to the needs of older bikes. Plastic pieces being the crutch for repairs. If they break, and replacement is necessary, time and cost goes up. Repainting may be necessary as well, which adds expense. How many people are going to be accepting enough to pay for it? Not many, I presume.
A dealer turning away work is due to avoiding the headaches associated with making a customer face the realities of older vehicle ownership. Many people don't want to face this. If my experience with auto shop ownership are lined up with motorcycle shop ownership, this is the biggest determining factor as to their decision to work on older stuff. 10 yrs old is, IMO, too early to deny service. Rusty bolts are a rarity on motorcycles. They are being overly cautious.
Technician experience may also play into this to an extant. Yes, newbie techs learn EFI stuff, and probably barely cover carbs in school. With time come the experiences of trial and error to make great techs. Everyone of us have unwittingly helped a newbie get his feet wet in some form. Especially in the medical field.