I think they're intended more as a deterrent for people that might commit a crime than a rehabilitation institute.
The fear of prison will not stop any of these criminals from committing the crimes they are accused of committing. They never set out to commit a crime. No Ponzi scheme ever starts out that way.
Prison can deter you from stealing granny's pocketbook. It will not deter you from stealing out of shame of exposure from failure reproduce the outsized returns that were initially delivered. The inability to overcome an inflated ego and admit that exact failure always leads to the ponzi scheme. The dream of recreating the returns at a later date fuel the continuance of the crime. No potential prison sentence will ever deter that.
There is no rehabilitation in prison. Prison is a punishment that society tells itself works. For murderers, rapists, abusers and the like prison is the only place for them. But for financial crimes its a waste of time.