No assistant gets any royalties even now.
I would propose the same people get royalties who do now. Let's say the director is assigned a 1/5 cut of the royalties - if he dies, the movie is that much cheaper to license. Until all the specified personnel have died - at which point the movie becomes royalty-free.
Then all artists get rewards from their works during their lifetime (which was the idea in the first place) and heirs get none.