This is the take I have too. I don’t think this is a matter of arrested development or “not growing up”; I’m suspicious of the prejudice behind such statements.

Collecting vs playing with toys is the difference between exchange-value and use-value. You can infinitely accumulate exchange value in the form of toys, but use-value drops off after a small number of toys. You only need one or maybe a few toys in order to play. All that has changed is the social acceptance of collecting and the mainstreaming of that hobby. I would guess in proportion as adults are unable to make “real purchases” (in scare quotes) on housing and such, because those are too expensive.

Adults have long been toy collectors. Before we had funko pops, Boomers were buying Barbies and other dolls.