First off, it's impossible to know that a child will spend their lifetime in an institution, because changes happen all the time. So it's an impossible scenario -- such knowledge can never exist, so speculating about it is just an exercise in abstraction.
But the reason to me is that if you never exist you will have no chance of happiness. If you do exist there is always a chance of happiness, even if the scenario ("living in an institution") creates an image in people's heads of no chance of happiness. Institutions of all kinds (traditional and nontraditional) are terrible places, but people do carve out lives wherever they end up in life and the lives they carve out usually involve the chance of happiness.
As for giving a child every conceivable advantage, I assume that you would also abort then if the child was anything other than white, male, heterosexual, cisgender, etc? Because failure to be that uber-privileged person causes varying degrees of disadvantage in life, including disadvantage that can be far worse than that faced by disabled people in many circumstances. So in order to have "every advantage in life," that's the sort of child you would have to have. Of course then the trouble becomes where do you draw the line. Every advantage could be construed as having much higher than average physical fitness and (valued-by-the-majority) cognitive abilities. And since the line between able-bodied and disabled is incredibly arbitrary, you'd have to work that one out too.
And yes, that's eugenic. That's sort of the definition of eugenics.
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"In my world it's a place of patterns and feel. In my world it's a haven for what is real. It's my world, nobody can steal it, but people like me, we live in the shadows." -Donna Williams