Check out this great article by Gene Park. I always enjoy Gene's logical and analytical posts on the CGC message boards.
I agree with Gene and as I've stated countless time on various message boards, the comic book buying demographic is dying. Today's 5 year olds will not be interested in Silver Age comic books 30 years from now. They aren't interested in comics now, why will they be willing to pay thousands and thousands of dollars for them in 30 years?
Once the current crop of serious collectors dies off or sells off in 20-30 years, who will be left to buy these books? Will there be enough demand to sustain prices? No way.
Certainly iconic books such as Action #1 and Spider-Man #1 will maintain value because they cross the "comic book collecting only" line into the area of pop culture / Americana, so they maintain a much higher level of interest from a larger audience. Non-key issues do not cross this line, and their value will plummet once no one is left to pay big bucks for the more esoteric issues. In 2038, very few people will care that Spider-Man #28 is tough in high grade because it has a black cover.
The rally cry of the die hard fanboys is, "As long as they are making movies, people will want the comic books." This logic is laughable. To today's kid, the Spider-Man franchise IS the movies and videogames. What makes anyone think that today's kids are going to generate a spontaneous interest in old funny books decades from now?!?