Not only that, but I imagine in pre-industrial times, those who got currently-curable forms of cancer didn't get cured during those times, and Darwinian forces

Not only that, but I imagine in pre-industrial times, those who got currently-curable forms of cancer didn't get cured during those times, and Darwinian forces stopped them from reproducing. There isn't a necessarily good or bad here but those Darwinian forces did eliminate a lot of genetic defects from the population before modern medicine. In the future though we might be able to yet again eliminate defects not by natural selection but by gene editing while still allowing those individuals to carry on a normal life.