> even this length of sentence has a high chance of producing a hardened, bitter criminal with hacking skills.

You present this as an argument for a shorter sentence. But from another perspective, it's an argument for never letting him out.

Prison isn't primarily meant to rehabilitate; you are almost certainly right that it will do the exact opposite in this case. Its power to deter is also limited. But what it can do, if we are simply willing to use it for that purpose, is contain dangerous people and prevent them from harming others again by simply not giving them the opportunity to do so.