Thanks! You ain't seen nothing yet!
Actually, compare it to military discipline, and what Gen. Patton once said about it.
Military discipline is something that gets enforced. In case of breaches against it, punishment follows.
According to Patton, ideally, dispiline should be so strongly ingrained into a soldier's mind, that he automatically acts according to it. Discipline must be perfect, it must become a habit, instead of a compulsion.
This would mean : less breaches, and hence less punishment. In the ideal, perfect world, there will be no more punishment, since discipline has become a habit. Thereby, the necessary deterrence of punishment is also ingrained, but since everbody has the habit to follow the rules, it is no more applied and hence physically absent, and ultimately, mentally too.
I apply the same thinking for the deterrence of crucifiwion (or legal punishment in general).