There is a cognitive decline. Not because of “overworking” the brain, but because of Auditory deprivation. Your brain is like a muscle and needs constant stimulus to exercise. With an untreated hearing loss, you are depriving your brain of that stimulus and it starts to break down. You will loose your ability to comprehend speech sounds in the frequency range of your hearing loss. Generally speaking, vowel sounds are in the low frequencies, consonants are in the highs. The longer the loss remains untreated, the worse comprehension becomes. It has been linked to Alzheimer’s and dementia.