Absent any proof that the objects were truly stolen, I don't feel any need to return something to someone in some country who suddenly finds an interest in getting something back. What does ownership mean to you?

Let's say you come to my country and buy a souvenir. Can I decide, hundreds of years later, that you must be forced to give it back?

And why do borders matter? The argument seems to be that housing an object on one side of an arbitrary political line is morally superior to putting it on display on the other side of some invisible line. Somehow someone born to the right parents is a morally superior curator compared to someone born into the wrong parents.