Addiction.The word is a fallacy.Just stop if you want to.
Well, "addiction" isn't a fallacy, but I'll admit this was my first thought, too: If you're worried about the amount of soda you drink, drink less.
If missing the caffeine gives you a headache, either HTFU or take Tylenol for a few days. It will pass. The thirst-quenching aspect is in your head, and you can overcome that by drinking water BEFORE you get thirsty. Spending money on bottled water is even dumber than drinking soda (clean, pure water runs out of taps all over America), but you can carry an insulated cup or refill a bottle (personally, I'm embarrassed to carry a bottle around--I don't want people to think I was silly enough to pay 2 bucks for it)