I agree with you Doc. As I said to my Harley collecting friend. In twenty years time when he wants out of his old bikes, the guy's who are his age will want out too. Who's going to buy them. The new generation of cashed up 40 year olds might be into old evo harley's from the 90's. Harley's might even be passe by then. I've owned Harley's since I was 18. When I was in my 20's there where a lot of guy's my age riding them. I don't see scores of 20 odd year old's riding them now.
I think this applies across the board to most makes of bikes.
Sure their will always be the guy who wants something from before his time and will pay the dollars. But there will be less of them.
I spend a lot of time trying to speculate the motorcycle market, as I have no interest in stocks or real estate.
Honda Z50's from the 70's pull good money. $2,000 for a tidy one. A Z50 from the 80's pull $600. In ten or so years the kids from the eighties will be on a mid life nostalgia trip and the price may go up. The other thing to factor is motorcycles boomed in the 70's. Hence why were all here. But in the 80's the market declined. So will there be as many of the next generation into them. I spent some time at a swap meet recently with some younger guy's who grew up in the 80's. They are into their cars and bikes, but they were chasing and collecting 80's BMX bikes and riding gear because that's what was big in their day. They had no interest in dirt bikes and were bemused at me buying old dirt bike junk.
But thing always change. I drive a Cadillac, most of the people in the Cadillac club are 50'+. But a lot of rappers feature old Cadillacs in their video clips and now and lot of guy's in their 20's are getting into Caddy's. I sold my last one to a twenty year old who bought it for his first car. But an old caddy is still cheaper than a used SS Holden. So for a young guy it's a cheap alternative, when his mates pay $30,000 for a used Holden and he pays $12,000 for a Cadillac with way more street cred. You also see a lot of young guy's cruising in old Holdens and Fords from before their time. But only because they are still cheap.
If the price surpasses the price of a late model used car they will probably drop out.
So you can speculate all you want, but like Doc says it's all crystal ball stuff, but with unexpected twists and turns.