Familiarity, Interest and Availability...
Years ago, when I started collecting vintage road bicycles, I published an article that I titled
VINTAGE ROAD BICYCLE COLLECTABILITY
The only thing that will change are the people and the kinds of bikes that they will be seeking. Who would have ever thought that the BMX bike would achieve collectible value? Not me, that's for sure! But look at Ebay today. Lots of vintage BMX stuff.
And, as for aluminum bikes, they too will have their following, as will carbon fiber(if any survive long enough). It all comes down to why people collect and that won't change in our lifetimes.
Anyway, for those who wish to do so, read through the article. Even I found it to be interesting and still, pretty much, on target.
Actually, one thing will change. I will be the only guy left with original paint and art on his bikes:-)