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?
Maybe that is better than our current system, but I think we should certainly aspire for better.
> "Having indeterminate sentencing would also be rife with possibilities of misconduct"
While that is true, I would argue that not incapacitating somebody for exactly as long as is necessary is itself injustice. It means that we will knowingly be imprisoning people longer than is necessary (obviously abusive) and it means that we will be knowingly releasing non-reformed criminals back into the public (I consider this to be abusive to the public.)
Incapacitation determined on individual basis is a precise tool, a very sharp knife. In the hands of an authoritarian regime, it could be very dangerous. But in the hands of a liberal lawful society, it allows the least harm possible for everybody involved.
We could deny ourselves these tools, for fear of what could be done with them (and in the process, cause certain unnecessary harm). Or we could strive to be the very best that we can be, and give ourselves the tools necessary to achieve that.
You write as if you genuinely don't know that USA imprisons a far larger percentage of the population than any other nation, now or at any time in the history of prisons. A humane person wouldn't even have to consider the grotesque and obvious faults of the system (racial disparities, entrenched prosecutorial misconduct, emphasis on victimless crime, etc.) to know that "pre-mature release" is the least of our problems.