No where in your quoted section is the OP saying that 31% of US births should have been eliminated.
He said that 31% of US births WOULD NOT have happened in the past.
Big difference between saying something would not have happened in the past, and saying something should have been prevented.
The OP is right in the fact that modern science is changing our overall genetic makeup.
Your wife fell off a horse. Maybe she was taught improperly, maybe horses don't like her.
In the past when riding a horse was a primary mode of transport, your wife would be at a disadvantage. If she fell off a horse and couldn't have kids that removes the can't ride a horse trait from the gene pool. Note that this could be a learned behavior. If your wife didn't learn to ride a horse properly, odds are her child would also learn incorrectly.
Had your wife not broken her pelvis and been able to have a natural birth, then the strong bones / knows how to take a fall trait would have been passed on.