hey anigbrowl, OP here.

You raise some great points - it is indeed impossible to use an MBC to bring sounds back into a user's range of hearing if they have lost all sensibility at that particular frequency. However, the loss of hearing at a particular frequency is not binary - it tends to start with a reduction in dynamic range at that frequency, as the cilia start to get worn out / destroyed.

So if you have loss @ 3 KHz, you don't often completely lose all hearing, but your dynamic range which normally is 0dB -> 100 dB (over-simplification here) might now be 30 dB -> 100 dB.

What an MBC will do here is compress the range at that frequency band, so your 100dB of range is now 70dB of range.