Is that the king on the throne?

They should return the pieces, it would be the right thing to do. As for what the owners of the pieces do ... that's up to them. It would be nice, but ... it isn't likely the Europeans are going to return all their plunder of their colonies or something ... still it would be a nice gesture. Even if they can't because then they'd have to return everything to everyone and the museum would be pretty much empty I suppose. It's not like the pieces would be going to another continent anyways ...