There is no legitimate "collective ownership" and none of these museums should return anything unless a specific owner shows up and can prove his claim - not likely for the Parthenon Frieze.
You mean if some foreign power, like say Iran, where somehow able to steal the Declaration of Independence, or the Liberty Bell, or some other thing, then they should be entitled to keep it? Those treasure hunters weren't trying to protect anything. In fact, their shoddy methods ended up destroying a great many antiquities, such as mummies. They weren't very pretty, or made of gold, so the treasure hunters simply dumped them in the desert. Don't defend the indefensible.
I know that instinctively, we think that noble Westerners are far more deserving to keep these treasures than miserable peasants who probably can't appreciate them at their worth anyway, but that is not a legitimate opinion. And, BTW, there is such a thing as patrimony.
The “museums” have made tons of cash showing, marketing, and quite often mischaraterizing the cultural and historical creations of other peoples.
At the least they should be returned! So I steal my neighbor's whatever and tell him that I'll keep it because his lifestyle is not conducive to maintaining the property I've stolen???
Pretty dumb argument...you're better than that.
I thought that Nicolas Cage returned all those after finding them in National Treasure.