darrylri
Cannonball Rider #52
Until I bought my airhead, I rode old Yamahas - a 1982 Virago 750 and a 1980 XS850. I was able to buy most parts from my local Yamaha dealer. Post sales tax and waiting 4 or 5 days for delivery, they were about as cheap as Internet parts, delivered sooner, and put money into a local dealer's paws. If 40 years is the line you draw, which is the far end of your own assessment of airhead ages, they my experience is of no use. But as far as dealers supporting old bikes - many do.
Actually, I'm limiting myself -- and BMW's support for their past products -- by ONLY talking about bikes 40 years old.
When my 1953 R51/3 was crunched in an accident, I bought a lot of parts for it from BMW, including things like a new rear fender ($600 in primer, not bad considering what plastic parts cost). I bought new spokes, valve covers, handlebar levers and their perches, footpeg rubbers and a variety of other parts.