At least LEGO is probably the toy that gets "passed down" the most, my own LEGO parts who I got from an older cousin, is now on its 4th generation (first my sisters children, then some family-friend to theirs), and I'm sure the pile(s) will get further passed down as time goes on.

True, but at least it's not single use. Is there a viable alternative? A non-petrochemical plastic that has the same qualities? It's not like they can whittle them out of wood or cast them with metal so it'll always be some form of polymer, and I'm sure they would jump at a more ecologically sound option.