Again, you are assuming he is guilty. What if he actually isn't guilty of what he's been accused of? How does he successfully defend himself decades after graduation, now that he no longer personally has the assignments he delivered during his studies and the professors who graded him are now dead or senile or simply don't remember him?
I'm not assuming anything. I'm saying that if the facts or evidence presented show irrefutably that a person withheld required information to advance themselves, then there should be no limit as to how long they should be punished for it. I'm talking about something major like a criminal history, not a minor thing like cheating during a highschool exam, or a parking ticket you never paid.