There are many more factors to consider than sentience when assessing the value of a human life. It is illegal for me to walk into a hospital and murder a brain dead patient on life support.

In the US, you could if you were the next of kin making decisions about care, but only by unhooking them and not ending their life in less potentially traumatic ways. Criminalizing euthanasia is another great example of unethical law, and it is legal in many places.

While I agree that the evidence seems to suggest that legal elective abortions affect some positive social outcomes, I am not convinced that that alone is a sufficient argument to permit legal late term elective abortions.

After 24 weeks there is a reasonable case to be made for this, however you might be interested to know that even after that point fetuses are kept anesthetized and sedated in the womb until first breath:

the fetus is actively sedated by the low oxygen pressure (equivalent to that at the top of Mount Everest), the warm and cushioned uterine environment and a range of neuroinhibitory and sleep-inducing substances produced by the placenta and the fetus itself: adenosine; two steroidal anesthetics, allopregnanolone and pregnanolone; one potent hormone, prostaglandin D2; and others. The role of the placenta in maintaining sedation is revealed when the umbilical cord is closed off while keeping the fetus adequately supplied with oxygen...
a massive surge of norepinephrine—more powerful than during any skydive or exposed climb the fetus may undertake in its adult life—as well as the release from anesthesia and sedation that occurs when the fetus disconnects from the maternal placenta, arouses the baby so that it can deal with its new circumstances. It draws its first breath, wakes up and begins to experience life. source

Eugenics also produces desirable social outcomes, eliminating genetic diseases and improving overall intelligence and physical health. This alone is insufficient to validate the practise.

As you mention, eliminating heritable diseases is also eugenics, the kind that few complain about and many people still practice by choosing not to birth children with genetic defects, selecting partners with fewer genetic risk factors, or simply choosing not to reproduce. These practices have not been invalidated and persist to this day. The eugenics that people object to is forced eugenics, which more often than not overlaps with genocide.