As I see it, one of the biggest problems with the American justice system is pre-mature release. We are releasing criminals before they are known to be rehabilitated. If we're going to go with rehabilitation, then we need to scrap the idea of determining how long somebody will be incapacitated (imprisoned) during sentencing. The duration of incapacitation should be determined solely by how long rehabilitation takes.
A doctor continues treatment for as long as treatment is necessary. You don't hear them say things like "Okay Mr Johnson, looks like you've got cancer. We're going to do chemo for 6 months, then you'll be done!" Rather, you get "we're going to start chemo, and we'll continue to monitor your progress."
We also need to set sane expectations for our own abilities. What is the false-positive rate for rehabilitation? Does that false-positive rate, when considered with the severity of the crime, work out to a risk that we are willing to accept? What if 1% of captured and rehabilitated murderers go on to murder again? Is that an acceptable risk? What if it's 5%, or 10%? Are there crimes so awful that no false-positive rate is acceptable?