Sickle cell disease affects millions of people, and it's the most common inherited blood disorder. We know for sure that certain mutations in the HBB gene cause sickle cell disease, so I think there is 100% certainty when you're correcting this specific mutation. People who don't inherit this mutation don't get sickle cell anemia. (Because their HBB gene doesn't produce blood cells that are shaped like sickles.)

I didn't realize that CRISPR gene editing can cause damage to other genes [1], so it might not worth it yet. If there was a perfect CRISPR procedure that had no other side effects, then it would be hard to argue against that.

[1] https://www.sciencedaily.com/releases/2018/07/180719165032.h...