The thing I think most law-abiding/normal people don't understand about the types of people in prisons is that they are almost exclusively habitual offenders.
Even in the US, which is heavily punitive instead of rehabilitative, you have to have run-ins with the law a number of times before you are sentenced to serve time, rather than probation or a halfway-house etc.
From my experiences, ~90% of people I saw during my time in jail and prison were coming in and out, constantly being arrested. There's no desire on their part to change, they're a lost cause.
For the tiny handful of people that genuinely do want to have a normal life and not go back to prison, our justice system is awful. No, solitary is not good for that.
You need supportive + rehabilitative programs. They won't solve everything, but for the small number of people who want to get out of the system, they need an honest-to-god out and chance at living.