Why do we imprison people? For two reasons. First, to isolate incorrigible reprobates from society so that they can’t harm it. Second, to reinforce the deterrent incentive or stimulus against those who are thinking of committing crimes, to show that the state is serious about punishing lawbreakers, to strike fear into the hearts of the morally lax.
Here’s why neither of the two reasons require actual people in prisons.
First, the very fact that some temperamental Monsters are in prison means that they were not deterred by the existing threat of punishment. Very well, what if we up the stakes? Let us consider an ideal situation in which everyone’s punishment is personalized to him. “Is 5 years in jail enough to deter you from stealing a car? No? How about 10 years? Still not enough? 12 years in solitary confinement? Also severe beatings every day? How about we’ll have rats devour your hands and feet? Also genitals?” And on we go ratcheting up the brutality, until our Monster trembles with terror at his fate and refrains from stealing the car.
If there is a chance one can avoid detection, the punishment is to be increased still more to compensate for it.
Second, ideally, the threat is credible to Barely Humans even without actual evidence (trials and sentencings) that the legal system works efficiently.
In an ideal world, then, prisons would exist but be empty.