Your statement makes some unsafe assumptions. Mental state has a strong effect on ability to reproduce and raise offspring. Cancer rarely has an onset early enough to be a factor in this and the traits aren't something that can be "sensed" by potential reproductive partners. Evolution is blind to debilitations that don't impact reproductive ability (this is the same reason our natural healthy lives aren't much longer, on average, than the time it takes for us to be successful grandparents).
Maybe in a few hundred years with a society that stagnated with the exception of a strong stigma being placed on reproduction with people pre-disposed to cancer you'd see a natural reduction in the traits.
Maybe in a few hundred years with a society that stagnated with the exception of a strong stigma being placed on reproduction with people pre-disposed to cancer you'd see a natural reduction in the traits.