To what end? To spend tax dollars? To make them rot away indefinitely?
What good are we doing to society if we are keeping rehabilitated individuals locked up at taxpayer expense? There's no objectively correct amount of punishment. The correct amount of punishment should be the smallest amount of time necessary to be confident that the criminal won't cause more harm to society, especially when the crime was committed as the result of a treatable illness like addiction.
To what end? Justice.
Simply saying "you did a terrible thing, and that's irredeemable" isn't useful to society. What good are you doing if you've rehabilitated the criminal? You're just spending tax dollars on principle. It's cruel and unproductive.