I think the choice of using the word “dementia” in the context of untreated hearing loss is a poor choice. But I don’t think it is an appeal to fear, nor is the loss of critical thinking the result of untreated hearing loss. An obvious evidence is simply looking at our deaf community and those folks are thriving just fine despite their hearing loss and I’m sure their minds are just as sharp as the folks in the hearing community.
I think the right way of saying how untreated hearing loss (if it can be treated with hearing aids) can affect our brain hearing is that the hearing information that is lost and untreated eventually will be erased from the brain’s memory. Not only that, the brain is very dynamic and efficient, and it will re-purpose/re-wire unused portions of its memory for something else if left unused for too long. So your brain hearing operates in a certain way when your hearing is normal, because all the sound information is there and available for the brain to work with and evaluate and make decisions on. But over time with untreated hearing loss, more and more information is lost, so the brain hearing has to adapt initially to the loss, but eventually at some point, the amount of information loss is so great that the brain hearing becomes less effective and unable to recognize long lost sounds and forgets how to process the long lost sounds anymore.
So let’s say that you start losing your hearing in your 20’s and it gets worse and worse but you don’t start wearing hearing aids until you’re in your 60’s. The brain may no longer be able to pick up all of the lost sound information instantly and know how to process it anymore after 4 decades of gradual loss. Sure, maybe if you keep wearing the hearing aids and continue to go through a long rehabilitation process, your brain may eventually be able to relearn itself and get back into shape with respect to its brain hearing power. But this would be a very long and hard road to recovery, if even feasible, because we all know that the learning power of the brain is more effective at a younger age but by the time you’re 60, it’d be much harder for the brain to learn new things.
So while associating untreated hearing loss to dementia is maybe too much of a generalization, I think there’s some truth to saying that untreated hearing loss will alter the way our brain hearing functions, and if left untreated for too long, we may not be able to get our brain hearing ability back to the way it was or the way it should be anymore because the brain has already re-purposed that ability for something else and it may be very hard to get it back, especially in old age.