OK, I hope my question doesn’t get misunderstood, I can see how that could happen.
Just a product of overthinking.
Idea is that we can live fairly easily even with some diseases/disorders which could be-life threatening. Many of these are hereditary.
Since modern medicine increases our survival capabilities, the “weaker” individuals can also survive and have offsprings that could potentially inherit these weaknesses, and as this continues it could perhaps leave nearly all people suffering from such conditions further into future.
Does that sound like a realistic scenario? (Assuming we don’t destroy ourselves along with the environment first…)
I expect gene editing soon to become so cheap that everyone starts customising their children, resulting in a situation analogous to where dogs are now: extreme variability improving the chances for survival by making sure we have the needed people for any situation except gamma ray burst which requires backups far from Earth.