Off topic some, but reminds me what one antiques writer, Rinker, wrote in a column many years ago. He said there were three things every collector hates to hear:
(1) why did you buy that?
(2) where are we going to put it?
(3) what am I to do with it after you die?

On topic, one reason for declining value that the author didn't exactly mention -- some things are not just out of style, they are not used or made anymore so collectors don't bother with them anymore -- the users of the things have died, and so have the collectors . Some example:
¶ board games like Monopoly, and card games are not played as much as before, so less collector interest over time.
¶ tin toys are not around -- so no one around to buy back their childhood.
¶ old trading cards with pictures of old movie stars (silents, for example) and western stars -- that memorabilia is probably worth less today.
¶ Stereo view cards -- had two pictures mounted for parallel viewing, on 7 x 3.5 inch heavy cards, usually curved slightly with axis along the long length. Good example of little interest today. Not used. so not collected.
¶ if chips are no longer used due to electronic record keeping in casinos, people might lose interest in casino chips 50 years from now.

Robert