While I agree with your general point -- that rehabilitation isn't determinable at sentencing -- I'd still make the argument that focusing on rehabilitation with no other change in the system would lead to more positive outcomes than our current system. Yes, keeping people in some rehab situation for as long as necessary but no longer would be ideal, simply allowing for some possibility of rehabilitation for the time that they are sentenced would still be a better situation than the one we have now. Having indeterminate sentencing would also be rife with possibilities of misconduct. For an historical (and I believe this still happens in some cases?) example of how things can go wrong, take a look at the old state-run mental health system the US had.