>A good chunk of harmful mutations will prevent people from procreating, either by killing them first, or making it unlikely for them to find a mate.

well, what about a recessive allele that doesnt cause problems in the first generation but does in subsequent? What about co-dependent deadly / devastating gene expressions between newly added genes that pop up after 2 people with these co-active genes expressing like a hundred years from now? look, given what i studied and why, i would never say we should forever abstain from gene editing technology. BUT, we have to hold those who conduct genetic modification experiments to the absolute highest ethical and engineering standards. It is akin to the level of power given by atomic science, maybe even more. one can be sure that the powers of a God would include the power of the atom and the power of the gene near the top of the list.

Not nearly as much in 2019's modern societies.

Less now, for sure, but you're overlooking a major category.

Many of the really bad mutations are embryonic-lethal, which is why a surprisingly high proportion of pregnancies "don't take" or end in miscarriage.

Yes, and many more cause carrier mutations that won't surface until the next generation, or late-onset metabolic or neural disorders that will cripple the individual later in life, or increase mutational load resulting in much higher chance of cancer. The fact that some mutations are benign and others fatal really doesn't change the issue that we don't yet know how to safely edit the human germline.

Totally agreed--I think this was a horrific and stupid thing to do.

I just wanted to push back a bit on the idea that medicine is badly "breaking" evolution by keeping people alive.