Is it not in every way an improvement, and in no way a loss if the child is born without DS?
Again, you are attempting to make impossible value judgments about other people's lives. This is literally the same reasoning governments have used to support their eugenics programs. But it is impossible to know the answer. You don't have enough information from an outside perspective, and you can never have an inside perspective into another individual's life, so you will never have enough information to make such a value judgment for anyone but yourself.
Your thoughts on how a larger disabled population can affect society at large are a value judgment I think can reasonably be made--of course, you won't necessarily be correct, but it is possible to make informed opinions about it, at least. I also appreciate that you are against the application of force to achieve a lower ratio of disabled-to-non-disabled people. However, I think that you might consider dropping the portion of your argument that has to do with presuming what would be best for individual Down Syndrome patients, their families, and hypothetical healthy replacement children. Because you are getting into extremely murky territory here.
Perhaps. It was really just for the sake of discussion, but I appreciate that for many families it is not some purely academic matter.