It's different because Coke is chemically addictive, albeit much less than the drugs we commonly think of as addictive.

Even though the caffeine is the same chemical in Coke or other drinks, when you become addicted you also feel a compulsion towards the other circumstances around your consumption, like the exact taste of the drink. It's similar to why people prefer particular brands of cigarettes or beer - the preference gets stronger over time as it's chemically reinforced.

Well, it's not stagnant. The formula is (other than the change to HFCS perhaps) but the marketing, which is the true product, is not. What New Coke taught is that people aren't buying a drink for the flavor, they're buying the physical representation of good marketing and the positive emotions that come along with it. They've never stagnated in that aspect.