My point is that the Rosetta Stone is not part of the cultural heritage of modern day Egypt. Neither is Stonehenge part of the cultural heritage of today's modern Britain. Would this be enough justification for you to cart it away?
Nor do I accept your premise, on which your conclusion is based, that everything that has ever been found, no matter when, within the geographical boundaries of modern Egypt, is the property of the "civilization" that exists there today.
Then by the same argument, you must therefore not accept the fact that Stonehenge belongs to the British people of today. You see, I would argue that it does.
I agree that Stonehenge belongs to modern England. But if the Romans had carted it away during their occupation of Britain, before there was an England, then I would not think that modern day England would necessarily have much claim for assuming ownership of it.