And his voice took a decline. It happens. he was older and had drug problems
Like an athlete, you only have so many jumps & bumps in you. A persons vocal chords only have so much singing in them.
His voice changed multiple times over his life b/c he'd been singing non-stop since he was probably 5 yrs old.
For most men in general, their voices get lower as they age, and singers often have to pitch their songs down a half-step.
His voice still had life even on Invincible, I just didnt care for the production that much. I wish he'd done more in between that time and his death, but at that point I'm sure celeb life, lack of sleep, drug dependcy had taken it's toll on him. Probably harder to concentrate and operate as he once did.