I think the idea is that there are many factors that might motivate somebody's decision that their life is not worth living any longer. Each individual will have a different threshold for that, but if there are institutions that can alleviate the reasons for the people on the fringe, then there will be net fewer people that choose death when they otherwise may have.

Someone with advanced dementia may be too far from that threshold to change the decision for them, but that doesn't mean that better institutions wouldn't move the needle.