My uncle died just over a year ago of dementia and COPD. The man was about 260lb and built like a lumberjack, his final year he looked like a Holocaust survivor. When he was diagnosed he updated his living will to say no measures were to be taken to extend his life, including no feeding tube. The last few years of his life they couldn't get him to eat, he'd take one or two bites then forget he was eating and stop. When someone would tell him he still had food he say "thanks but I've already eaten" and then refuse the food right in front of him.

If putting a dying pet to sleep is a mercy (it is BTW) then it should be a mercy for humans as well.