I think this is very close. I'm no biolooo gist, but I think that what would eventually happen is that there would be three steps of treatment. First one to repair the structural damages/degeneration with genes such as nt-3, second to transform the supporting cells into hair cells with atoh1, and third to act as an enhancer such as with fgf20 to further grow the hair cells. For all I know the atoh1 gene therapy hair cells could be incomplete.
I think that focusing primarily on the deaf may be a mistake. So, if they work out how to insert the viruses without further danger, they should consider working on people with better hearing because they would have less degeneration, which I believe would mean more success. Eventually those that are worse off could benefit from the breakthroughs.
What I hope is that hearing loss can eventually be reversed and there are enough treatments on the market for it to be more acceptable for government to implement a law where any headsets or earpieces above 80 dB require a prescription. I don't think this would do much harm on the market since such devices aren't indestructible.