Virtually all of Dan Klein's objection was based on the idea of comparison with the past.
This paragraph from his post sums up his objection:
Designer babies would attenuate coherence with the past: One hundred years hence, people would say, "When you watch him on the old videos, he may not look like much, but back in the old times, Mike Tyson was considered a pretty menacing fighter." People in the new times would not know our sense of standard. And they would have difficulty knowing it. Everything preceding the break, from Achilles to Rafael Nadal, would be foreign and unintelligible.
We're not even sure that's true, given the trickiness of genetic engineering. But let's say it were true. Is that a problem?
Actually, although Bryan Caplan stated his objections to this point very well, another commenter on Dan's post stated the basic objection even more succinctly.
So the cons are that everyone would be so amazing that they wouldn't appreciate how less amazing people were in the past?
One thing I can think of is similar -- with the advent of vaccines, people don't even remember how bad diseases were, and therefore may sometimes not have their kids vaccinated. Do I wish that vaccines never happened because now people don't have an appreciation of historical disease outbreaks? NO! I'm happy most of them aren't dead.
Or think of it another way. Think about a technology that has come along that has not made babies stronger, more competent, or whatever, but has made our lives much easier. That also would attenuate coherence with the past. I'll give an example that I often used when I gave talks about the IT revolution in the late 1990s and early 2000s.
When I was a teenager in the 1960s, long-distance phone calls were very expensive. As a result, they were very rare. When we got one in our household, everyone got quiet so that my mother or father could listen carefully. In fact, I remember one such phone call in about 1964. My father said, "Shhhh" and we shushed. We learned that my great aunt in Peoria had died. Now, when we get long-distance phone calls, we often hang up on the caller.
In Dan's terms, the change in phone call technology with the resulting greater than 95% reduction in price "would attenuate coherence with the past." In my terms, it doesn't attenuate coherence with the past. It just makes me greatly appreciate the present.
Or take cell phones. When I used to land at an airport in another city and have a friend pick me up, we had to make sure in advance exactly where to meet. If he got stuck in traffic or if my plane landed late, that was a problem. The usual way to handle it would be for me to find a pay phone (remember those?) and call his home and hope there was someone there whom he could call when he found a pay phone. I need not tell you how we do it now. I don't miss those days at all. And, by the way, technological change plus deregulation have caused airline fares to fall (although not lately) making my flights much more frequent.
Or, fill in the blanks. Think of your own example of a technology that makes the past look unimpressive because it makes your life so much easier now.
Coherence or attenuation of coherence with the past just doesn't do it.