Cancer is a very poor example when talking about eugenics since 1) most forms of cancer are non-hereditary and 2) cancer also rarely requires lifelong treatment

Cancer is a very poor example when talking about eugenics since 1) most forms of cancer are non-hereditary and 2) cancer also rarely requires lifelong treatment, instead you either get cured or die (it can resurface though even when you think you are cured). And for eugenics to work you do not need to die, just not get any kids, which is why Sweden used to have people sterilized. One of the strongest arguments against eugenics is that when it has been applied in practiced it has brought out some of the worst sides in humans.