Yes I feel a rare comic book will always keep its value and most of the time increase in value as time goes by, but a rare comic book must not be confused with a so called hot comic book, these books are ones that are popular for example around the time that character has a movie release or a major event happening in the comic book around his character, thus for a short time that characters comic books will show a strong increase around the time of the event but then its prices will drop away again when the hype surrounding the event begins to die down
I believe that like anything collectible. Rare comics can be very valuable. Their worth is based on the condition of the comic and its scarcity. A wise investor will pay what he feels is right for his circumstances. The final worth of the comic can vary greatly depending on the number of people bidding. The final worth of the comic is up to the bidders
Considering how popular Spider-Man, Batman, The Avengers and all the other comic book characters are now I think it's very plausible for the very rare, early comics to be worth a lot of money. I think millions of dollars is a little too much but definitely thousands of dollars easily.
Yes, rare comic books are worth thousands, and potentially millions of dollars, because something is worth what anyone will pay for it. If people are really that much into comic books, the demand curve will raise the price of something high if it is very rare and a lot of people want it. It's an economic issue.
I collected comics during the comic glut seen in the 1990s. Yes, during that time, many comics were worth quite a bit of money. However, with the exception of a few stand outs, like Action Comics #1, Detective Comics #1 and other first appearances of notable comic book characters, the back issue market, even in rare comics, is in decline.