You create embryos, which are distinctly uniquely a human, then destroy the ones you don't want. It's eugenics, just at an earlier stage of human development.
It’s the difference between a man dying of old age in his sleep vs putting a gun to his head in his sleep because he is old. As to percentages, 100% of people die eventually. Just because people die every day doesn't mean it’s ok for us to kill them.
I wouldn't be sure if it really matter whether embryos can suffer or if they deserve full human rights with right to a legal representative. To me the photos of them look like cells under microscopes, frankly. But our instincts do alert us that doing this could kill us all somehow, and for that reasons we hallucinate such moral arguments. Isn't that so?