Why should we try to stop the inexorable forces of natural selection or survival of the least looserous. Do we really want idiots living long enough to spawn?
Because we are not, on the whole, heartless psychopaths who can look upon human suffering, even humans who are not immediately connected to us, and just shrug our shoulders. Our nature is to be social and empathic. Natural selection is cruel and uncaring.
If I think you are an idiot, should I be allowed to prevent you from breeding? If not, what will stop idiots from breeding? Certainly not natural selection.