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…)
There are already lots of great answers, I would like to point out that Natural Selection doesn’t care about the individual at all, it cares about the population, e.g. internal gestation, do you think any individual enjoys carrying a baby inside them? Preventing them from doing anything during the gestation period, being an easier prey to predators, etc… Unfortunately for the individual, creatures that carry their unborn babies inside them are less likely to abandon them even temporarily while seeking food, they’re also more easily kept warm, so for the species as a whole it’s better that there be internal gestation.
In short more individuals = better, imagine you have two populations, one with only 10 strong individuals, and one with 100 individuals of which only 10 are strong, which do you think is more likely to survive? And that is even assuming a strong/weak deterministic position, which is not the case for anything.