Would telling them "Your child is stupid because you are stupid!" really be more successful than pointing out a lack of effectiveness? And would they really ignore the left-over variability? And will they stop at Baby Einstein CDs or will they just neglect any IQ improvement method, like regular and complete school attendance?

You're missing the point entirely. Imagine if people thought the same way about height as they do intelligence. Neurotic parents would waste all their time trying to make their children taller. The nightly news would be filled with segments on how X or Y impacts your child's height. Yet, none of this would do much to change outcomes, because height, though highly variable among siblings, is mostly genetic.