Damn. Wonder if it's something that can be treated.
Since it's neurological, I am sure there is some therapeutic stuff you can do, but like all stuff... Alzheimer, Dementia, ALS... it's progressive. I am sure his more recent movies have hidden those signs, but it's just getting to a point where it's hard to work. Judi Dench's eyes have gone bad so much, she can't read scripts anymore.
With Bruce, it's speech. I am sure he could work without saying anything, but at the same time, Bruce probably said it was time to retire and not trying to hide it anymore.