While I agree with your assessment to a degree there are many factors that determine collectibility. One such factor is the age demographic of the collector and the general interest in a certain item.
As we get older we either lose interest in a particular brand or type of bicycle or worse yet we leave this earth for the next generation of collectors to pick up where we left off.
There are many niche areas for collecting even in the road bicycle category. This allows many to enjoy the hobby at all price levels. While someone may like an old Schwinn road bike another may distain it. The more people that focus on a particular bike drives up the value and interest. This combined with relative availability makes one bike more coveted than another.
I contend that as we age and move on, those niche areas may evaporate in interest but the real desirable bikes have more longevity and will attract the newer collectors. They may not be the best bike made in their day. They may not be the best looking or built bicycle but something draws many to them as a moth to a flame.
Legends and stories add to this mystique and general value. Dare I say that a Rene Herse will be a popular bike among collectors for many years to come. I am not so sure one could say that about the majority of the bikes discussed on this forum.
I have seen this same phenomena in my other hobby that I have left dormant for some years now. Record collecting. Much of the music of my generation is considered old and passé by the following generations. Still there are a group of dedicated collectors that will always want an original Sun record by Elvis or a Beatle butcher cover. If you go to any record convention, everyone in attendance, young and old will know of these. I am afraid the same cannot be said for Gerry and the Pacemakers. Although I love their music and have all of their records. The young folks could care less or may have never heard of them in the first place.
Youth shall be served.
Ray Homiski
Elizabeth, NJ