What about doing the same for Autism or even something like diabetes? Both of which would create life-long complications for the child.
As an autistic person, if I could trade my life for a world without autism I'd consider that a small price to pay. My life is not more important than the happiness of future generations.
It seems more like selecting certain types of consciousness as acceptable/unacceptable which is problematic to me.
This is a good point. How can we evaluate which types of consciousness are needed? After all we live in a world where the technology exists to support a wide variety of neurodivergent subpopulations. However, that technology exists because of conventionally intelligent people. They are what makes the industrialized, first world conditions which support DS individuals possible.
In light of that fact it would seem wise to ensure that there are always conventionally intelligent people around to keep that support infrastructure working, and to improve it over time. We cannot yet produce people with unusually high intelligence at will, but we can maximize the odds of their birth by maintaining a large neurotypical majority from which such individuals may arise.