A 68-year-old man presents with gradual onset bilateral hearing loss over 10 years. He reports hearing speech but struggling to understand consonants like "S" and "T". Audiometry shows bilateral symmetric sensorineural hearing loss with preserved thresholds at 250–1000 Hz and progressive loss at 2–8 kHz. The structure marked **D** in the diagram indicates no air-bone gap. Which of the following best explains the sensorineural nature of this hearing loss in presbycusis?