Random House:
1 - any nourishing substance that
is eaten, drunk, or otherwise taken into the body to sustain life,
provide energy, promote growth, etc.
From reading the multiple entries, I don't think the definition of food is dependent upon all aspects being true. As soda contains at least one essential nutrient (sugar/carbs), which is used for at least one vital process (energy storage), I would argue soda qualifies as a food under any of the definitions provided.
Which leads to your first point: necessity or luxury. Of course, soda by itself is not necessary, but then again, neither is bread. They both supply the body with essential nutrients, but can be substituted with another "food" containing the same nutrients. Considering we need a balanced diet, most "foods" taken individually are not necessary. No one needs a banana or a steak or milk, we need the nutrients in them and a combination of the foods to get all the different types of nutrients.
This could be important for two reasons, first because a tax such as this makes any and all foods susceptible to the same or worse laws. As long as a "better" alternative is available, any one food can be regulated at will. Secondly if science ends up creating a "new food" which includes all the nutrients and none of the potentially harmful chemicals, laws like these will give the government the ability to mandate citizens to ingest only the "new food", as it would be the best alternative of all.
I would argue that soda is as necessary as any other individual "food": taken by itself it won't keep you alive, but in the proper dose and in conjunction with other "foods", soda can be a viable source of energy without harm to the user. One among many alternative sources of energy available to the public, so that any of the individual substitutes becomes a luxury. Why tax sodas but not energy bars?
That gets to what I see to be the heart of the matter: the status of soda. Drinking it is still perceived as a bad habit, one mostly engaged by youthful/immature people and therefore an easy subject for legislation (kids can't vote and adults are taught to "know better", even the ones that like the stuff). The science does not justify this, as coffee would be a much better substitute for action, containing the same and many more chemicals including rodent carcinogens. Although I'm not sure, I would think more people drink coffee than do soda, so the health benefits would be greater in taxing the former. Furthermore, we should question why we apply a sin tax, while we could just as well mandate the elimination of caffeine from all sodas, theoretically leading to a reduction in use. Regardless, taxing soda and not other substances is simply arbitrary and justified only by "someone's opinion" and what's politically feasible. If the sugar in soda is the problem, why aren't we simply taxing sugar?
So too is the preference for a long and healthy life "someone's opinion". I'm fine with the government providing information and protection from others in order for individual's to extend their lives as long as the individual wants. Given that longer life leads to higher costs for society, I don't see providing it against the will of the individual as an obligation for government. To force people to abstain from making certain choices, only because it is better for their own health is an opinion, not at all a moral imperative.