IMO:
If trial design was theoretically perfect, the drug should be better for moderate sufferers for a variety of reasons.
- Higher likelihood of more in tact support cells
- Hearing loss usually disperses over time from high to low frequency from noise-induced (most common cause) hearing loss. FX-322 can really only hit 6-8 kHz at the very max.
- Correlative factors: People with severe hearing loss are more likely to have multi-factorial problems such as synapse damage, genetic factors, viral or autoimmune factors, stria and/or vascular problems. They are also more likely to be older.
Optically speaking, there's a nice sell for the moderate group, which is that if the drug worked, they may be able to avoid the hassle of hearing aids. On the other hand, even if FX-322 worked as intended, there's no way it's replacing the role of hearing aids in people who have severe or profound losses.
It does seem, though, like "proof of concept" in vivo could be easier to demonstrate in the severe group (particularly patients with mostly just standard hair cell loss) because it's not the same random chance of the drug properly targeting the right lucky patch of inner hair cell loss before it gets diffused away.
I think it was smart of them to cast a wide net in order to make sense of these considerations.