I can see no reason, moral, economic nor practical why they should be returned.

Might as well demand that all LS Lowry paintings are returned to Manchester or that London Bridge be returned to London.

Auguste de Choiseul-Gouffier removed part of the Parthenon Frieze before Lord Elgin and sent them to France where they now reside in the Louvre.

He had exactly the same permission as Lord Elgin to do so and I have not heard about any demands to return these antiquities.