I have a couple of points to add:
1. As a horror fan, I've found that a combination of the internet plus uncut, restored Blu-Rays and DVDs of older movie titles has served as a great gateway into older films. Granted, someone still has to want to watch them, but I believe that many of the older films are more accessible today than ever before and therefore can be seen by a wider audience than ever before. Oddities and Z grade trash films that were once classed as obscure are now easily purchased, so I feel that this may help keep the hobby rolling along.
2. Many collectors covet items that the average person would be amazed about - matchboxes, salt and pepper shakers, medieval pottery - so I don't believe that the relative time gap between a movie's release and now necessarily equates to lesser demand. Are there a lot of people who would want a daybill for 'The Sinful Dwarf'? Probably not - but there will be some, just as there were 10 years ago and just as there will be in 10 years from now.
3. Price will always be related to demand. Time won't always automatically equal price rises. I've got daybills I bought 20 years ago that you could get for the same price today. With my horror collection I'm always after what's rare, not necessarily what's famous. I've had what I class as the key posters in my collection for years, nowdays I'm more excited to see if I can find 'Black Belly of the Tarantula' than 'Suspiria'.
If it can be classed as collectible, you can bet there'll be someone who'll collect it.
