> since the 1990s children—millennials—aren’t collecting like generations before them.

I would say (being a 1989 baby), that it isn't due to a lack of wanting, it's due to the systematic mass production of anything and everything. There is nothing worth collecting anymore, because there are 10 billion other copies of that thing... I mean, there are now more than 1,000 Pokémon when there were only 150/151 when I was a child. My holo-Charizard isn't worth anything these days.