It will start off very slowly. I'd argue we're already doing "genetic engineering" on a small scale with IVF. At some point, the IVF clinics will start offering "screening" for incrementally less harmful diseases, disorders and traits. And then it's a small step from that to simply picking "the best" embryos. E.g. They'll "filter-out" all the embryos that will end up being very short. Then they'll filter out all the embryos that are "most-likely" to be below-average in intelligence, or most-likely to be mentally challenged, etc.

Even if it isn't down-right engineering/splicing/etc, and using only the parents' egg/sperm combinations, they can still offer to "move our society" forward at a snail pace.

I personally see no reason to be against it, as it's the logical extension of natural evolution. It's just more refined, scientific, guided, and devoid of some of the randomness or trial-by-error properties that are inherent with natural evolution.

I completely agree that a little bit of caution is important. That's not at all what the precautionary principle is. It says that before you do something, you must first prove that it's harmless.

This kind of proof is of course impossible, and in practice it means "never do anything".

The fun thing about is that people only advocate applying it to things they don't want to do, never on their own favorite projects