I don't think you will see our niche hobby plummet. As long as there are movies and people to see them, there will be collectors of movie posters. If they stop printing posters, like I thought would have happened 5 years ago, the nostalgia factor will probably kick in and send more folks into the hobby. Paper stuff, rare, fun.
What I think will happen is a shift in what people collect. When older collectors die, some of the stuff that they were so fervently collecting will probably die with them. Some actors from the 40s and 50s will be forgotten. A handful will remain in our collective consciousness. Some new trends will appear, others will vanish. When I started collecting in the early 2000s, I accumulated a lot of 80s posters, the era I grew up in. They were dirt cheap, no one collected them. Now my generation look at them with nostalgia and they're as a whole worth more than x10 what I paid for them. One day in the future, the 2000s will be hot, and the 80s will be something of the past.
Let me give you another example. I also collect XIX and XX century art. XIX century was red hot 40 years ago, now it has cooled down a lot. The old Masters will always be at the top, but a lot of average work that was selling a few decades ago is now dead. People view it as grandma stuff. They are more interested in contemporary and modern art. And so it goes. With the Chinese crash, contemporary art, which is way too high, will most surely take a gigantic hit in the coming years. A lot of names fall off a cliff, a few survive and we all move forward.
T