The Aaron Swartz analogy doesn't work. The point that the parent was trying to make was that for your average violent and even non-violent criminal (typically low-income upbringing, living in run-down conditions, living on the "streets" but not in a homeless sense), prisons are not real punishment. In fact, conditions in prison can for some be an improvement. Aaron grew up in an upper-middle class environment. Prison would have been a total change for him.
The problem with deterrence is that criminals by their very nature do not think about the consequences of their actions. This is a generalization, but criminals tend to have poor impulse control and don't think about the future. This is why many studies have shown that the death penalty does not work as a deterrent - a murderer is not being rational in their decision to murder someone. They either do it in the heat of the moment (can't control their temper), or they do because of overwhelming emotion/hate/spite/sociopathy (premeditated murder).
Here's the flaw in your argument against the necessity of deterrence:
If a policy effectively prevents a targeted category of people from becoming criminals, then most criminals won't be from that targeted category.
Say p ~ 10% of people either don't think about the consequences of their actions or don't care whether they go to jail. Say q ~ 90% do care about the consequences of their actions and do care whether they go to jail.
Under a system where deterrence is a strong element, most criminals will be from p. But remove the deterrent, and you'll start seeing more criminals from q.
Let me clarify. I did not say that deterrence is utterly without use. When I said that there is a problem with deterrence, I meant that the current state of deterrence in our justice system is broken. There is a mistaken belief that more punishment necessarily entails greater deterrence, which is many times not the case. There is really a threshold when it comes to deterrence. There is very little to no deterrent effect of the death penalty vs. life without parole, as others have posted here. In fact, I personally find it hard to believe that a life sentence versus, say, 50 years in prison, has any additional deterrent effect for something like murder. The rational 90% would still be adequately deterred.
The problem with deterrence is that criminals by their very nature do not think about the consequences of their actions. This is a generalization, but criminals tend to have poor impulse control and don't think about the future. This is why many studies have shown that the death penalty does not work as a deterrent - a murderer is not being rational in their decision to murder someone. They either do it in the heat of the moment (can't control their temper), or they do because of overwhelming emotion/hate/spite/sociopathy (premeditated murder).