Well, from my admittedly "selection-biased" perspective, I *do* know some elderly people. Yes, the ones with dementia are quite miserable. The ones who get into a really severe state, generally don't "last" more than a year. (thankfully). I also know a couple of elderly people (in their 90's) (and, I've had some relatives, as well, up in their late-90's) who are totally mentally sharp. They have hobbies, activities, and some health problems, but nothing horrible. I don't even know how people like this die. :)
I've also known a couple of guys who, in their 70's, got cancer . . .
And I've known a couple of people who suffered from strokes, and heart problems.
Unfortunately, strokes and heart problems can lead to dementia SYMPTOMS. (until the heart just quits, of course). Take care of your ticker, and your blood pressure.
Of all these; I'd say cancer was probably the easier death.
But my first choice is "none of the above, and have a happy, full-life into my 90's". Whether I have any friends or family or not.
Dementia would be last on my list of "ways to go peacefully". Maybe 2nd to last, because ALS fucking sucks too. (just ask Stephen Hawking).