I'm not convinced prisons are still necessary. The technology is high enough that people can be isolated electronically, virtually, using house arrest. I don't see prison as being necessary except for the most dangerous, such as terrorists and people who are set out to destroy society. Most prisoners aren't set out to destroy society and aren't violent.
My point is consider the level of surveillance we get put under? If we are building a panopticon why do we need to keep prisons when that is looking like the current and future?