The prevalence of Down syndrome on average is not too high - on average, from 3 to 30 per capita, depending on the region (there is a direct relationship between the well-being of the region and its prevalence). Those people who are born with this syndrome in any special way, in fact, do not affect the picture of the world. Often these people remain able to work, are able to serve themselves and can be employed in simple jobs. The reproduction of individuals with this deviation, of course, leads to an increased risk of having children with the same deviation, but the risk remains in completely healthy parents. We will identify and screen out children with this syndrome using the method of negative eugenics - their number in society will be minimized. This is taking into account that parents in some regions do not have the opportunity / desire to conduct prenatal examination. Again, this will not affect the overall picture of the world - the percentage of this deviation per capita is still small.
I was interested, in fact, why the author of the question is interested in this particular syndrome. Indeed, in addition to him, there are many other deviations - moronism, idiocy, mental retardation, physical deformities - hundreds of various disabling syndromes from A to Z. Not to mention congenital, "endogenous", mental diseases, which are not detected at all at the stage of intrauterine examination. ... If we try to identify all of them - then yes, it will work, the percentage of the healthy population on the planet will increase significantly, let's omit the errors, which always remain.
I can add that I was somewhat confused by the statement of the previous speaker, with whom it is fundamentally not I agree. The mechanism of transmission of intelligence by inheritance is still very little studied, although it has been proven that the genes responsible for intelligence exist and work. For example, genius is not transmitted at all in any way - this property arises spontaneously, if you will, also as a result of mutation, often closely adjacent to a number of nervous and mental disorders. So, it can be said offhand that, even at the stage of formation of obvious disabled people, it is quite possible to increase the percentage of children with a "diagnosis" of "practically healthy" and increase the general intellectual potential. Real intelligence, in practice, depends more on the conditions, lifestyle and upbringing of the individual. In unfavorable conditions, the potential runs the risk of remaining potential.
At the moment, human civilization does not produce evolutionary natural selection, evolving exclusively using intellectual and sports development, forgetting that it misses the direction of self-sifting to remove a natural defect.
How does it work? By canceling defective units in the next generation we we get a smaller number of sick people. We can go further - the unification of all people with an intelligence below average and, in general, the percentage of smart and genius people increases. Artificial evolution is taking place. That is, with the correct selection of a subhuman from a person - the less we are, the better we are ...
There is a double-edged sword here: often people with down syndrome get a job, they can have offspring (though 75% of cases when paired or one with Down syndrome, or both, that they will be born down), even in the cinema filmed (e.g. Chris Burke). So how did these people hinder society? When this pathology is revealed at the stage of pregnancy, parents, of course, are not sweet, but this is not the case when a completely vegetable is born, which is collected by all Channel One for treatment by SMS messages, so if parents are adequate people, not constrained by stereotypes and opinion of society, then they can bring up quite a good person for themselves. In general, I advise you to read: 8 successful people with Down syndrome.
I think for the better. Already, there are techniques that allow at the embryonic level to identify this pathology and terminate a pregnancy. Although this is a genetic abnormality, it is not inherited. Therefore, some time after the termination of pregnancy, the couple can again conceive and give birth to a healthy child. If you use this algorithm of behavior, there will be more happy married couples in the world, although it will be among those people who are not strictly religious.