modern medicine allowed misfits to reproduce?
Its not really complicated. Defective people who reproduce are more likely to produce defects. Women giving birth after 30 are more likely to give birth to defects. Simply put, smoking, alcohol, drugs... all the things which increase probability of defects, when applied a lot on a mass scale end up with giving more defects. Same goes with vaccines. Remove natural selection and the defective ones reproduce as much as fit ones.