And the idea with stolen art is that "dirty art" should theoretically always trade at roughly the same discount to "clean art," and in fact, over time, as gaps in the provenance become easier to patch up through fabrication, the discount should reduce. In that sense, "dirty art" is an investment, so the immoral of the super-rich are fine holding it forever. Also, no need to pay estate taxes if it's already an undisclosed asset on a yacht somewhere.