I also wonder if they simply CAN'T do what's best for themselves anymore. I heard another cliche' "You can lead a horse to water, but you cannot make it lie on it's back and float" This may be what happens? I personally feel for them. On the otherhand it is not beneficial to them or us to enable them to carry on in their self destruction.
I have never been very good at "Tough Love" but sometimes even tiny steps might help I think. Trouble is that when one is in the thick of it all it is very difficult to maintain any sort of detachment. I believe this is where peer support ie. AA and or Alanon can help keep things in perspective and give a bit of distance. Sometimes it takes more than going to one group to find the right fit. I have been to several different Alanon groups over the past 25 plus years and only 1 group "felt" right. It was an immense help. Prior to that I found that Forums like SR were also good because this way we see the stories and the experiences and are able to come to our own conclusions without the distractions of the "feelings" of fitting in.