Rave, there is already a disincentive for alcohol consumption on an aircraft: it's called personal responsibility. Passengers are in close proximity to one another in an airtight tube travelling at 500MPH. It really should be up to passengers to moderate their alcohol intake.
Why would someone drink to excess on a plane anyway? If people get drunk it ruins the flight for fellow passengers and cabin crew alike.
Anyway, I totally agree with you that airlines do not necessarily help by offering alcohol during their trolley service. However, the assumption is that passengers know how to drink in moderation. Cabin crew will decline alcohol to a passenger that appears to be drunk. This is where many air rage problems begin: a drunk passenger wanting more alcohol.
Most of my neighbours are BA cabin crew as I live close to Heathrow. I hear this from them on a weekly basis.