A lot of the theories here suggest that "hard asset classes overall" (including fine art, coins, comics, etc) might drop 20% to 50% as a whole, at some point in the future.
I don't necessarily disagree with that.
But I'd suggest that that notion alone, is not damning of this specific book's recent run up (AF15).
If Golden/Silver/Bronze comics as a whole, drop, say, 33% across the board on average, that doesn't in and of itself imply that AF15 is currently overvalued relative to other high value comics from different eras (e.g. compared to Hulk 1, Hulk 181, Superman 1, etc)
The run up in comic values as a whole may indeed be mainly related to ZIRP, overheated economy, etc.
But the "extra" recent run up in AF15, is, IMO, a reflection of the supreme popularity of Spider-Man compared to other (even very popular) Silver Age superheroes. In my opinion, even at its prices of 2 years ago, it was an undervalued book relative to other Silver Age keys when you consider the characters' relative global popularity.