I think they became worthless ironically because they were so treasured. Previously the good China may have had some special occasion remembered, some social importance where they would be brought out. Being so guarded they were never used and took up space and thus never acquired any memories for the newer generations, just being a social cargo cult tautology of valued because they were valued.

Toys in the original unopened box may be worth more but that isn't why there was demand for them in the first place. Unless there is some sort of hilarious misprint, defect, or very questionable concept (accidentally looking very sexual, looking like a reference to some notorious event) thaf makes it a curiosity in itself, the toys were valued because of their use.