I am not commenting on the appropriateness of someone continuing to have children with a serious genetic disease. I would like to point out however, that the life of a child with CF can be horrific torture at times, even with medical advances. The life of the parents or caregivers for that child is also extremely challenging and stressful, because watching a child struggle to breathe is unbearable. Even when the child is doing fairly well, there is always the threat of an imminent attack. It is the nature of the condition.
No one can tell someone to have or not have children. But seichel suggests to me that if someone knows for certain that the odds of a child having a hereditary eventually terminal illness are great, that person should receive proper guidance both medically and halachically, as to how to proceed. We are not allowed to be somchin al haneis. If 7/10 kids have CF, it stands to reason that any other children to follow are more likely than not to suffer from the same.
There is a reason why Dor Yesharim was established. Should those behind that program be told that they are wrong to try to prevent tragedies before they arise? I think we need to be better educated, and to encourage people to know their medical history well, so we can try to stem the tide of genetic diseases becoming more prevalent. the best and easiest place to start is BEFORE a marriage takes place. Hashem Yeracheim on all our children, that they should be healthy and live long lives.