This is interesting, especially Doc's comment on the generation thing. I also think that high prices will drive a lot of people to cheaper alternatives. I was keen on getting an egg yolk yellow SL70 because I was on a nostalgia trip as it was my first bike. Now that there $3,000+ do I still want one just to scratch a nostalgic itch. Uuumm .. no. I saw a mint unrestored one in the states and have the photo, that's enough.
I was talking to a guy at a swapmeet recently about Kawasaki Z1900's. He wanted one but the prices where too high, so he bought one of the new Eddy Lawson replicas for less money. He said it go's better than an old Z1 and when he gets nostalgic he just goes and looks at a Z1 which is what he was doing when I spoke to him.
I've recently had a big debate with a mate of mine about this topic. He collects and restores old Harley's. He's fastidious on using original parts. No repo stuff for him. Even if he builds a '48 custom he will only use genuine period correct custom parts that where available in '48. I told him that I wonder how the value will hold in the longterm future when the guy's who are into them now are too old to care and the next generation wont care or wont and can't pay the big dollars. Most of his bikes are worth around $30,000 to $50,000 each. He got a bit upset and told me it would never happen, and that if he sold his bikes he would ship them to Vegas and put them in the auction to catch the big fish. But what happens when everyone sells their old bikes and flood the market with them?