from a few years ago in the New Yorker-- probably the best take I've ever read on the issue (link below). It's not always old people-- but it is, often, people who objectively probably should just be allowed to die. It's hard to make those decisions, but the cost and pain to the person who hangs around are also very real, and so you have some pretty big forces acting against each other on this.
https://www.newyorker.com/magazine/2010/08/02/letting-go-2