This is a good point, it can be hard to predict what gene does what. Most genes that have been targetted for gene therapies tend to be the singular and known so

This is a good point, it can be hard to predict what gene does what. Most genes that have been targetted for gene therapies tend to be the singular and known source of a genetic disease. Something like the sodium transporter mutation in cystic fibrosis. If you fix it, it is clear what will happen; you will clear out the disease. Most diseases are not so simple and we are still getting to grips with even mapping the mutations scattered across the genome and the global human population that are relevant to certain diseases. I think when you dont have a single genetic cause, but a mix of various genetic factors instead, then the drug route is optimal. For exactly the reasons you stated.