But you are right that they have not stagnated on Coke's marketing, although those Santa Claus commercials have been pretty repetitive.
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.
They way I see it, Coke's goal is: 1) to make you want a Coke whenever you're thirsty (that's the marketing), and 2) to make it very easy for you to get that Coke (that's the supply chain/logistics part)