> and if, experimentally determined, the risk of negative side effects were the same as, say, postponing childbirth by five years,
To determine that some change is safe during the expected 150 year of life, you must measure the effects during … at least 150 years (preferability more).
How would be you sure that there are no tradeoff? Perhaps it reduce the chance of reproduction by a 1%. Perhaps it affect the ability to climb very tall mountains or very deep scuba diving. Perhaps it will make your grandgrandchildren more weak during a famine (sooner or later, some of your descendants will suffer a famine) or an epidemic. What are the good genes for surviving a war? How do you average the advantages and disadvantages of the normal times and the war times? And there are some adaptations that cause problems but makes you more resistant to malaria, how long you expect your descendants to live in a malaria free zone?
How are you sure that the proposed changes are real advantages and not just some fads? (Consider that it's not clear how many eggs should you eat weekly, the recommendation changes every ten years.)
What about exchanging 10 years of life for a 1% more chance of reproduction. Perhaps 1% is small, but it accumulates during the generations.
You can imagine an infinite set of negative consequences of practically any action --- your choice of breakfast cereal, via a chain of consequences, might cause the end of the world. Still, we make choices, because we have to terminate the chain of hypothetical situations at some point, using the heuristic that without a specific reason to believe that some effect happens, it doesn't.
With respect to the topic under discussion: absent any particular reason to suppose that this kind of gene editing causes your hypothetical wartime probability difference or your hypothetical 1% fertility drop, there's no reason to use the possibility of these effects to ban the treatment, since there's as much reason to believe you're making these things less likely as reason to believe that you're making these things more likely.
Regarding changes being advantages: do you really believe, deep down, that being weak is just as useful as being strong? That being resistant to disease is just as good as being vulnerable? That being dumb is just as good as being a genius? Give me a break: I can't seriously entertain the idea that we don't know which traits we'd want our children to have, given a choice.
To determine that some change is safe during the expected 150 year of life, you must measure the effects during … at least 150 years (preferability more).
How would be you sure that there are no tradeoff? Perhaps it reduce the chance of reproduction by a 1%. Perhaps it affect the ability to climb very tall mountains or very deep scuba diving. Perhaps it will make your grandgrandchildren more weak during a famine (sooner or later, some of your descendants will suffer a famine) or an epidemic. What are the good genes for surviving a war? How do you average the advantages and disadvantages of the normal times and the war times? And there are some adaptations that cause problems but makes you more resistant to malaria, how long you expect your descendants to live in a malaria free zone?
How are you sure that the proposed changes are real advantages and not just some fads? (Consider that it's not clear how many eggs should you eat weekly, the recommendation changes every ten years.)
What about exchanging 10 years of life for a 1% more chance of reproduction. Perhaps 1% is small, but it accumulates during the generations.