I don't think that an actual Long Island Iced Tea was ever 100% alcohol. It always has a sweetener (sugar or simple syrup), some lemon juice or sweet and sour mix, and some Cola for color and more sweetening. Yes, it is kind of like one of every basic spirit behind the bar but it is also sweet and easy drinking, whereas just gin/vodka/tequila/rum/triple sec on ice would be way more fiery and way less popular.
You should finish the other half because that stuff is way too good to waste!