The answers are not obvious. Arguably, putting anything into the water supply is seriously ethically questionable, whether it changes only your hair color or lowers your IQ or raises it, for that matter. People have the right to accept or deny medical treatment. For treatment which occurs before birth clearly they cannot do that, but if you were meditating upon whether to apply a procedure or not, and you had adults who could understand the question and to whom the procedure would have been applied, taking their opinions into consideration on the subject is entirely valid.

You think of a person with Downs' as less than a person without it, clearly. But why should your opinion matter? If we accept treating Downs' in utero, should we accept genetic treatments to lower criminality? What about independent thinking? What about other "inconvenient" personality traits. Like why not allow some "authority" to eliminate any "negative" trait they wish from the population?

Obviously these are extremes and your position that considering the question with respect to Downs' leads to a straightforward conclusion: on balance, it make sense, but I think we should approach any question about modifying people with serious consideration.