The Elgin marbles were sold by the Greek government at the time. It was a legal sale. To come back 250 years later and want them to be given back is ridiculous. That is akin to anybody owning a Picasso must be willing to give it back to Spain because of origin of creation. Trying to make laws retroactive is a dangerous path indeed. Germany decides that military items of rarity and origin of country can force any that owns these items to return them because of cultural history. Same thing with ancient Japanese samurai swords.
Just an opinion.