Thinking of the C&V bikes.
Road Bikes:I bought my old Colnago in 1982 for 300,000 Lira, or about $200 to $300. It probably was similar to $1000 "new" bikes.
If it hadn't been used for the last 30+ years, the value might have gone up a little. But, even in pristine condition, it might only be worth that same $1000 or so. And if it was any other brand, maybe topping out in pristine condition at about $800.
In many senses, that is way under valued. A top of the line new bike is $5000 to $10,000 (or more). Just getting a retro-styled custom steel fame brazed together could easily cost in excess of the value of my "vintage" bike, not including the cost of all the components to go into the build.
Someone asks on the forum about going steel... and no doubt someone will give them the advice to hunt down a top of the line vintage bicycle, or vintage frame for the build.
I can see some of those true classics road bikes becoming rarer, but few are heading to the dump, and we'll still pile on top all of the 90's an 2000's used bikes. Perhaps the only thing to boost vintage bike value will be if we end up with a huge gap in vintage bikes, and loose a generation of carbon fiber and aluminum bikes. We did also have a generation when there was lagging interest in road bikes.
A recent renewed interest (I think) in road bikes may help the classics a bit.
MTBs:This will be an interesting phenomenon. I know the college campuses were innundated with $1000+ MTBs through the 90's, and I'm still trying to figure which of the junky bikes that are showing up today were those really expensive top of the line college bikes.
Nonetheless, there were so many mediocre MTBs from the 80's and 90's, that I don't see their value as booming. Somewhat like the high-end boutique store Schwinn Varsities from a generation earlier.
No doubt there will be a few classic MTBs, perhaps the Carbon Fiber Trek Y-Series MTBs.
Yet, then the question will be whether they will be desirable for their primary use? Will someone choose to take out their late 90's or early 2000's CF Trek, and go tearing up and down mountains on it?
On the road side, I have no problem taking my nearly 50 year old bike out for a 100+ mile road ride, although it is finally getting a break from riding in the rain.
I suppose one might argue that the primary purpose of MTBs was always as commuters, and the CF Y series bikes would do just fine on the pavement.