Well, personally, I wouldn't have had more children if the first was suffering, and would continue to suffer throughout a shortened life expectancy. But that's a very extreme case, and if you start talking about who should or should not have children (or more children) because of the risk of genetic conditions, then you get into a very tricky area, as it depends on what levels of risk and what levels of disability you personally would count as "acceptable".
No-one is perfect, and we all have a risk of producing children with genetically-cause disabilities.
My DH has a disability which may be genetic, and we knew before we had a child that he/she could potentially be affected. We thought about it, and decided the odds were good enough, and the condition minor enough, that we would go ahead.
I don't think anyone apart from the couple themselves can decide what is an ok level of risk or seriousness.