I think to quit a bad habit depends on each individuals, and what works for them.
Some people will do better by quitting at once while others will need to quit gradually.
It also has to do with the kind of the bad habit, like for a chronic smoker might need to quit gradually in other to be able to deal with the withdrawal effect of smoking.
Yes, to an extent it depends on the habit you have developed. Some habits like smoking and drinking are very hard to quit. And that's why these require extreme measures like forceful quitting not only from the think but detachment from the brain as well.