Looting is wrong, looters have no legal title to the things they steal, and the descendants of those looters do not gain such title merely because a half-century has closed behind the theft. And the same goes for what the Germans stole from Russia. To claim otherwise is, in effect, to legitimize looting as an outcome of war and to deny the validity or permanence of ownership. Maybe some of these objects would have been burned to ashes or blown to pieces if they had remained in Germany. And maybe not.
Nobody knows, and the question is irrelevant to the morality of keeping war loot.
At least the Russian museums have made an essential first step in revealing what their government had previously concealed. They point out that it was a response to Germany's attempt at cultural genocide. The Versailles Treaty, they argue, contained formal provisions for reparations from Germany to Belgium, to compensate for the enormous destruction wreaked by the Kaiser's troops on the country in 1914. They also cite the Allied Control Council at World War II's end, which endorsed reparations for war damage. Now Russian nationalists are getting into the act, demanding that their government stand firm on the issue and give nothing back.