[IanCal]

Would you edit a large legacy system full of the most horrendous spaghetti code, written in a language the best experts only slightly understand, doing many jobs that aren't documented anywhere which all depend on each other? A system with no tests whatsoever?

Oh, and you have to do it in production, and it's a critical system where people die if you get it wrong.

[guard-of-terra]

We know a long list of genes that are obviously broken versus a non-broken version that serves billions of people well.

Why not replace all faulty genes with known good ones? In homozygote cases, you can replace one of two copies with good version.