There's a difference between (1) securing a conviction on the basis of evidence which was compelling at the time of the trial but was, perhaps many years later, proven to be faulty, and (2) doing something improper to secure a conviction you know is wrongful.

Prosecutors should not be punished for (1) but should be punished for (2), in proportion to the seriousness of their misdeed and the sentence handed down (or served if they own up before the prisoner is released).