IMO more people should make the decision your relatives made. In the situation you describe, choosing the chemo would almost certainly make their life worse: the chemo wouldn’t work, and meanwhile instead of being able to stay at home, having the best possible last months of their lives, enjoying their families and whatever else they can enjoy, they’re being tortured by the chemo. They would be giving up too much for the small chance of a few more months. Instead of having the few more months now, they would be trying to have the few more months after the chemo-- but probably they wouldn’t get them.
Obviously this reasoning doesn’t apply to the 17-year-old with a very curable cancer. Most likely she has many years of life ahead of her.