This is the case but it’s also expensive to develop the cells used for AAV based drugs because the cells are killedwhen the virus is made. This, you can’t reuse the same set of cells throughout the process.

This is where orphan drugs come in. The government (US, anyway, but I'm assuming others have similar programs) can help provide some funding and allowances for private companies to work on drugs for conditions that have a low enough prevalence as to make it economically non-viable. Should someone that has a one in a million disease be left to suffer because Big Pharma can't turn a profit? This is a primary example of why and how governments should function: to ensure people aren't left behind. And, who knows what other things may spawn off such research, so it's not like everyone else might not benefit, too.