I agree. There are many diseases that would really benefit from this.
I fear you are right, and that some diseases would benefit by our preventing other diseases. Given that our focus is usually on the "worst" diseases, on average the competition opens to more benign diseases, but there will be exceptions. And some relatively benign diseases that seems easy to cure might become a target for a genetic "quick fix" that might, unbeknownst to us, open up for other diseases.
The interaction between different diseases and genetic "flaws" is not well understood, but we know there are interactions. Like, for example, how sickle cell anemia gives increased resistance against malaria. There are likely a lot of genetic conditions that cause ailments that were introduced because they also gives an advantage, which at an early point was a net win for some individuals. We don't have the full picture yet, so I would say there is a risk, and especially if treating relatively "benign" conditions.