My 82 years old father passed away from Parkinson's last week. He was an agricultural engineer on a communist collective farm working with pesticides and herbicides in the days when you'd have a teaspoon of DDT as breakfast nutritional supplement. Appears like Parkinson's is some 600% more prevalent in population exposed to herbicides. He has 7 brothers and sisters, none got Parkinson's and 6 are still alive (oldest one 87) but only my father worked with chemicals. Anyhow last years were tough and my parents are utterly unprepared for the inevitable. Like my 2 brothers, none of them acknowledge the fact that "dying peacefully by natural way" is akin to winning the Superbowl Lottery. And if they do, they're convinced they gonna be the winners and not the losers who get tortured and writhe in agony with severe psychosis while shitting their diapers for years until they die.
Your perspective is modern, when your father was a boy life expectancy there was probably about 60 and when his father was, 45 and down it went to 35 and below for men in centuries past. It was even lower in places like India and China for all their bullshit traditional medicine. Parkinson's it appears can be exacerbated by exposure to chemicals and it has a genetic component though not too commonly inherited. It's a perfectly natural way to die, its just that until recently few lived long enough dying of other things long before PD developed. The husband of my aunt spent his last 15 years on oxygen with fibrosis he believed came from spraying without a mask for just one week in the 1960s. It can seem and be cruel we have a great ability to prolong life fairly mindlessly.