I'd say, moderation becomes impossible at the alcoholism stage of alcoholism.
The problem with questions like this is, they presume there's some point when you know you have a potential problem and can make a wee adjustment to the amount or the occasion or style of drinking. That's not the way alcoholism works. An alcoholic either doesn't know or doesn't care about the problem until it's too late -- it's gone way too far to makes adjustments.
Some continue to lie to themselves that this or that will change things, but once the mental pathology takes hold, it's not much use. Then you admit that the only hope for a longer or better life is by quitting.