The family dynamics get really messy with this. I have an uncle who regularly expresses to me his desire to "get put to sleep peacefully like the family dogs" rather than be terrified all the time and waiting around to choke to death (late stage Parkinson's, and everything that can be is done is being done). My response is always, "I'll be the first to make that drive with you, but not while your brother is alive." There are some types of dog owners who as the dog ages will hit point where they're no longer keeping the dog alive for the dog's sake, but for their own. It's a bit like that, except they're not in a position to make that decision for them. But they'll do it all the same.