I had this reaction too, but if an offender is given opportunities to become a more conscientious and educated human being through hard work, it seems reasonable that we would want them released (and contributing to society in a positive way) sooner rather than later.
That already happens. "Released early for good behavior", early parole, etc.
Federal time is different. It is day by day. Only way you can get time off of your sentence is by going through drug classes.
Parole exists, which makes prison times extremely flexible, The exception is mandatory minimums, which are generally only for murder and for handling drugs that non-white/rich people tend to be disproportionately involved with.
I was thinking about maximum times, not minimums. There is no opposite of parole to extend their sentence, that I know of.