An addiction forges neutral pathways. Like a dirt road through a forest, the more frequently it is traveled the more deeply rooted it becomes. When an addict stops traveling the same road it will slowly become overgrown. There will always be evidence left of the road. It will always be easier to travel that way than a route that was never cleared of boulders and debris.
Another metaphor is that leaving an addiction is like leaving your hometown. Your memory of the roads will slowly fade. But if you ever go back it's not gonna take long before you're right back to your former proficiency.
I have known addicts who have returned to their addictions after a decade of sobriety. There's a reason in group they always identify themselves as addicts. It's a reminder to never be too proud. If they become lax, if they get proud, if they stop doing the healthy behaviors that got them sobriety in the first place their addiction will be there waiting for them.
The average number of stints in rehab for substance abuse is 7. Too often the addict dies before they figure out how to stay sober for life.