Tony's right, it's most likely to be a declining number of new, younger collectors joining the hobby.
That's why it's important to do what we can to nurture the next generation of collectors and inculcate a love of the hobby, and the things associated with it. Get your kids involved, if you have them, and get them interested in history. Toys and scale models are also good stepping stones to collecting figures. If you don't have kids, you can always join a club, or start one, to promote interest and fellowship in the hobby. Of course, you can do both, too, have your own kids, and join a club. The Internet is marvellous, but we still need to get out and meet other collectors, and see figures and toy soldiers up close.
Above all, be optimistic about it. If you just grumble and say, "No one's interested," well, the end comes that much sooner, doesn't it?
Prost!
Brad