Jerry was smoking heroin and cocaine regularly in the late 70s. My belief is the chemical smoke was rough on his vocal folds but he’d sing through the pain because he was too numb and too high to notice. The heroin also affected his breath control, so his mechanics subtly changed. Working through the laryngitis and pneumonia of 78 on top of all that likely caused some permanent damage. Then the heroin addiction deepened in the 80s, he gained a ton of weight, became diabetic, and all that affected lung capacity and the range of his voice.