What does it mean that breaking bad habits requires replacing not just stopping?

You cannot delete a habit — you can only replace the routine while keeping the cue and reward.

Example: Every afternoon at 3 PM, you walk to the vending machine for a candy bar. The cue is the afternoon energy dip. The reward is the brief sugar rush plus the social interaction with whoever else is in the break room. You try to stop — willpower, sticky notes, guilt — and it works for four days until Thursday when the dip hits and you are standing at the machine before you consciously register what happened. What works instead: at 3 PM you walk to the break room (same cue), pour a cup of tea and chat with a colleague (same social reward, comparable ritual), and the candy bar craving resolves — not because you suppressed it, but because you gave the loop a different routine that satisfied the same underlying need.

Try this: Pick one habit you want to change. Write down the cue (when and where it fires), the routine (what you currently do), and the reward (what craving it satisfies — be honest about the real reward, not the surface behavior). Now design three alternative routines that could respond to the same cue and deliver a comparable reward. Try each alternative for two days and note which one most fully satisfies the craving. That is your replacement candidate.