If a company has a monopoly on ALL soda (Coke, Sprite, 7-UP, etc) and prices go up too high, people substitute with drinking milk or juice.
A hypothetical for the sake of discussion: What if the same soda company also had a monopoly on the dairy farms, orchards and even the distilleries?
They can drink water, or wine, or anything else. It's unlikely that a single company will own all drink able products, because due to them being so different, a company trying to be the jack of all trades will end up being horribly inefficient, and a more focust comtetitor will emerge.
Also, not buying the product is a form of substitution (substituting it for nothing). Didn't want to go into it in the OP, but if soda becomes too expensive, people will buy it less, meaning drop in demand, and thus drop in price.