I for one think it will stay pretty constant since guys (and girls) love things that have wheels on them. I'm 29 and pretty new to the bike craze, but I love buying and fixing these old bikes. I teach my 3 year old son about bike parts and how to repair them and use the tools neccesary for building a bike. I teach elementary school and you wouldn't believe how many kids don't even know what a spoke is:eek: It's a great hands on hobby that in it's simplist form doesn't take many specialized tools or skills to do the basics but it's also one that can get specialized if you chose to explore things like painting, wheel building, hub rebuilds, etc. I enjoy it, and while I have no money to keep most of the bikes I come across, I atleast get to own a piece of history for a little while, get my hands dirty, and teach my son about bikes, tools, history, and things of that sort.
It's also great that there are different kinds of collectors to diversify the hobby. I'm in to ballooners, but you also have the middleweight, lightweight, TOC, pre war, guys as well so it's a vast hobby with many different paths to explore. Makes it fun!