If we standardized soda to be delivered and sold as high concentration syrup only to be mixed with water right before use, it would dramatically cut amount of plastic wasted per ml of final mixed product.
Soda fountains exist, are common, and predate plastic bottles.
Even if they didn’t, I’m not sure the logic here. Drinking soda from a bottle is just as much an unnecessary luxury as drinking water from a bottle, in both situations a person could just drink water from the tap instead. But we’re going to allow people to harm the environment with unnecessary packaging as long as they also include unnecessary sugar (which also harms the environment), but we won’t allow the unnecessary packaging if it doesn’t have the unnecessary sugar?