I'm not sure if you tried this, but your hypothetical is quite close to a real category of people. Young adults with down syndrome (a chromosomal disorder) have an average IQ of 50 and in developed countries typically live to only 50-60.
They used to be treated very poorly, sometimes exceedingly poorly. In the modern era they are generally treated with more respect than they were historically, despite almost everybody having lower expectations of them.
While people will take this as a sign of progress, I think realistically people with downs or other significant handicaps were a much greater burden on society in the past, and I think our current attitude of acceptance has more to do with our increased ability to care for such people.
As cruel as it seems to us now, if you're struggling to survive and there's no social safety net, infanticide or child abandonment start to become much more attractive options in comparison with being burdened by taking care of a dysfunctional human for the rest of your life.
They used to be treated very poorly, sometimes exceedingly poorly. In the modern era they are generally treated with more respect than they were historically, despite almost everybody having lower expectations of them.