This is Gene's complicated way of saying that everything has a ceiling. Since I'm not as smart as Gene let me give my slightly less sophisticated version... because this is a topic we debate all the time on the boards.
Everything has a ceiling and eventually has to correct. yes, from 1960 to 2003, comic book values rose substantially. And there are 100 other factors to consider as to why it did. What's the primary reason the hobby was able to drive up the prices? People with money pursuing comics, which are available, the old ones, in limited supply (especially in grade). Will that condition continue to exist as the years continue. It's not the supply of books which is the question... we can reasonably calculate that... but will demand still exist? Much harder to say. As collectors with these amazing books start to die or sell off, will there be new collectors there to buy them up? Do people really think these comics are worth their current prices?
You can't say that the sky is the limit. That's basically what you're saying when you say, comics have constantly gone up: If you looked at Microsoft when it was first released during the first 10 years, you'd say hey, the next ten years look pretty good? No, that'd be rediculous... you know better. How come when it comes to the comic market you put the blinders on and think it's somehow different.
Your books aren't going to be worth pennies tomorrow... but don't invest today thinking of sending junior to college tomorrow.