If I recall correctly part of the origin of soda was doing the reverse, from efforts to actively substitute for alcohol. Hence why they emphasized as "soft drinks" as a marketing category.

As for other substitutes, we've seen highly sweetened coffee drinks as the common one, to the point where soda taxes started to face accusations that they are actually thinly veiled classism due to exclusion of more white collar "Starbucks" type drinks. Rightfully or wrongly they have a point about it at least looking bad.

I wasn't intending to imply that is what would happen. That is the worst substitute I can think of. If high sugar coffee would also be counted as soda for the laws - something that isn't a given - this seems even more likely.