My family has a history of dementia and it's terrifying. Currently my grandmother is 95 and more or less can't remember anything from the past 30 years. She doesn't know who I am when I visit because I don't look like I did 30 years ago.

She requires 24/7 caretaking and most conversations with her end with "I don't know why the lord hasn't taken me yet". She's very physically weak and struggles to just walk around the house, etc.

This has been more or less her state for the past 15 years. Despite the best efforts of the family she is 95 and ready to go and not sure what else we can do.

I suppose what I’m really saying is that dying of natural causes and living in the frailty of old age really sucks. I’m not advocating for unhappy to kill themselves. I’m saying making an informed decision about when you’d like to die is ideal regardless of your emotional state. It makes sense for happy people to do this too. And maybe the answer is as long as possible, but I doubt it for most cases.

Because it's not about society; it's about the body (and mind) failing.

Why not both? You're assuming there is happiness for everyone, and they're assuming that everyone gets to a point where happiness is unattainable.