You bring up an interesting point in mentioning that art may need to be removed from an area that is experiencing conflict that may destroy the art. Should this become part of a global agreement when joining the UN? If the artifacts are part of our human heritage as a whole, are we all more responsible for protecting them than for claiming them?
I also agree that artifacts should stay with the descendants as much as possible, it is generally rude to take things from people. But I don’t know about handing them back to people who have moved away from the land they left stuff on… But yes, always be polite.