You have hundreds of varieties of bacteria in your mouth, many of which already make ethanol. You have one that makes lactic acid and causes tooth decay; by making a variety of it that makes ethanol and can outcompete the tooth-decay causing variety, you improve the situation: drastically lower lactic acid and not much more ethanol.
When ethanol is added to saliva, bacteria in the mouth produce acetaldehyde, which is known to be carcinogenic. I need less ethanol in my mouth, not more!
