There is an interesting discussion on statute of limitations I rarely see brought up over this sort of thing. Where is the line drawn, most rich societies around the world have been acquiring or have acquired things from others, I use acquired because some of this stuff was bought, some was stolen, and some was traded, and pretending as though this weren't the case is overly simplistic.
Further when we look at richer/more powerful individuals, organisations, or nation states acquiring items, by any of the listed methods, is there a point in time at which we'd judge it to be fine or moral, certainly there's no suggestion tribes in Africa who sold off slaves to european traders should also be among those paying reparations to their descendants, nor is there the perception that spoils of war should be returned nowadays for those nations, however it seems to be that this higher standard is only applied to a few wealthy nations, and only in some specific spheres.
I don't deny the moral argument for their return, however opening this avenue of thought leads to a pandora's box of possibilities on who "owns" what, and though in a utopian future one could imagine that all possessions were under collective ownership of the human race, and to be made collectively available, but I think we are a long way off achieving such an ideal.