At the beginning the effect of designer babies will be unclear. It might take two decades to see whether they are much more capable than other humans.

Different designers are also likely to try different ways to enchanced humans. Even if every human is enchanced that doesn't mean that there are no differences. Some designers will argue that humans should be able to produce their own Vitamin C while others will consider Vitamin C production superfluous.

There will be a trade off between experimental design where the result isn't clear beforehand and safer approaches.