The great dilemma of comic book publishers is that they are not building a new audience. If I am not mistaken, the average new comic book buyer is in his mid to late 20s. Sure, some teens and younger read the monthly pamphlets, but not nearly enough to sustain the market once the older fans drop off.
Certainly comic book licensing has never been stronger with the movies, video games, etc... but that does not necessarily translate into people buying comic books.
Many of the fanboys on the various comic message boards seem to think as long as Marvel is pumping out movies, their comics will continue to grow in value exponentially (as we have seen in the past 5 years with the emergence of CGC slabbing.) My response is, "If today's 15 year old isn't reading comics now, what makes you think he will suddenly want to drop THOUSANDS of dollars on an old Spider-Man comic in 20 years?!?" Sure, there will be SOME demand, but not enough to sustain insane price levels. The analogy I like to give is, "I like James Bond movies, but I've never had the urge to collect first editions of the original Ian Fleming novels."
I think the same will apply to comic books in 20 years. The movies may be chugging along, but the people who cared enough to collect the original comics will be entering retirement homes, dead and/or looking to sell their collections [to a tiny market]. Someday when our culture moves on to the next "big thing", comic books will be what old movie postcards are today -- crammed into dusty shoeboxes at your local flea market.
I think there will always be demand for iconic issues... pieces of Americana such as Action 1 and Amazing Fantasy 15 because they have crossed the line from being of interest to comic collectors only to being of interest to collectors of all sorts of pop culture and Americana. There will always be rich businessmen who will have an interest in displaying the the first copy of Superman in their office or den. Conversely, in 20 years will anyone but hardcore comic geeks -- remember, this is the audience that is dying off -- have an interest in Ms. Marvel 18, the first appearance of Mystique? (a CGC 9.8 copy recently went up to $1,225 on eBay and didn't hit reserve!) I can't think of a worse long-term "investment."
I love my comic collection, and someday I hope to get a few dollars back. But I don't delude myself into thinking that there is a 5 year old walking around now that will pay big bucks to take these things off my hands in 30 years.