What is advantageous in keeping anything that is guaranteed to need life long treatment to simply stay alive and is genetically accurately identified in our gen

What is advantageous in keeping anything that is guaranteed to need life long treatment to simply stay alive and is genetically accurately identified in our gene pool (eg cystic fibrosis, not mild myopia) ? Technology for this is already available (and even parental testing could already help diminishing incidence) yet I doubt it's common.