Ugh, I feel for you. My kid also has had to deal with her father showing up drunk to events. Fortunately he tends to be a pretty quiet drunk (but embarrassing in a quiet way, like trying to chat up one of the other moms who's clearly not interested). I can see my kid keeping one eye on her father whenever he's around and only giving half her attention to whatever she's supposed to be doing, which is pretty sad.
I share your frustration with "what does it take ...!?". My experience has been that unless the alcoholic repeatedly put the child in direct physical danger, a full custody change is difficult. I'm dealing with this with an ex who has two drunk driving criminal charges, repeated hospitalizations and police calls, and at least one instance of trying to drive drunk with his kid in the car (fortunately, bystanders prevented his car from moving). My lawyer said that judges see so many instances of kids whose parents starve them, beat them, etc., that having a parent who is "just" an addict doesn't seem like that big a deal, especially if there's a second parent in the picture who can pick up the pieces/act as the safety net/deal with the addicted parent. So the better a job we do as parents, the less likely it is that the other addicted parent will be regarded as a major problem. It sucks.