It's incapacitation, deterrence, and rehabilitation. Some add a fourth, punishment. But that's just really a means for the other ends.

Your position serves none of these purposes.

Rehabilitation: Making an exception does not serve a rehabilitative purpose. So a fail.

Deterrence: Making an exception not only doesn't deter, but it leads others to believe that their conduct will be excepted too. So your position fails again.

Finally: Incapacitation. By making a sentencing exception that doesn't serve the other two purposes, incapacitation is the primary function in your scenario. But you want to shorten the sentence too, which doesn't incapacitate a harmful person, someone that you've failed to rehabilitate via your chosen exception. Thus, You fail again.

Please don't don't ever write law or be a judge. 😒