Quite apart from the image issues and the potential to turn into a booze cruise ("I paid for this, I'm going to get my money's worth out of it"), what usually happens in such situations is that the quality of the alcohol goes way down to compensate for those people who are trying to get their money's worth out of it. You can do it in small venues and keep good quality, but large all inclusives tend to lower the quality to compensate for the quantity.
The worst experience I had was at my niece's wedding at an all inclusive Riu resort in Jamaica. We were staying elsewhere so we had to pay to spend the day there and for dinner. While we were waiting between the ceremony and dinner, one of my family members who was staying there said that she was going to the bar to get a drink and did we want anything. DH says "yes, a nice glass of white wine" to which she responded: "you can't have both - you can either have white wine or you can have something nice". Boy was she right. Tasted like it was made in a dirty bathtub last week. Disgusting. The rest of the drinks there were no better including at dinner. I ended up sticking with ginger ale. Went back to our own resort after dinner and had a genuinely nice glass of white wine.
I would hate to see DCL go that direction. Anyway, it would just push the price of the cruise up to cover the costs. If they were to keep good quality, it would push the price up even higher and people already complain about DCL prices. And not everyone drinks. So they'd be paying for other people to do so. The resort that we stayed at in Jamaica was all inclusive including alcohol but had a maximum capacity of 24 and was significantly more expensive than the Riu. For larger venues like a cruise ship, I'd rather pay by the purchase and simply include the price of drinks into my vacation budget.