Wow... this is a heck of a post! I am blown away by the level of detail you went into for this but for this topic, it's exactly what was needed! The social psychology aspect of this is a fascinating subject.

From personal experience, I am a pretty big autograph baseball, game-used memorabilia, and certain baseball card collector. This section of collectibles I feel differs from others as if you have a game-used or signed jersey it can double as something that is art/used for decoration. I think that taps into millennial minimalism since you kind of get two things for one. In times like today where so many things are mass produced like cars (unless you are going after the often way too expensive for the average person vehicles) collecting those is going to fade away until they are so old they are just hard to find in like 100 years. Baseball collecting is also something my dad really got me into and something we would travel to Spring Training and tag team to get autographs we wanted.

That emotional part of it I feel is something that really can't be replaced since there is an emotional (and thus a chemical bond in your brain) reaction. Beanie Babies might be the best example of people not understanding collecting and what makes it so unique. Something like coins that are minted with errors could continue to be collectible with future errors because the whole system is so automated and double and triple-checked.

Scarcity for the sake of scarcity I don't think makes something valuable there has to be a story/reason and a community behind it. Sports have the sports world for example. With coins, I see a different reasoning since FDR took the US off the gold standard and made citizens owning gold coins illegal whenever there is a mess up by the mint people want it for the sake of making fun of the government and highlighting its screw up. Not to mention so much of the US coinage was made with either silver or gold prior to 1964 there is a value in those minerals themselves.