That's a values debate, I guess. What is the purpose of punishment? Is it to set an example to others, is it to remove a dangerous person from society, is it to prevent the criminal from reoffending, is it to satisfy society's desire to see wrongdoers punished?

He didn't reoffend (as far as we know), and in fact went on to become a highly-contributing member of society. His crime was not so egregious that recompense was impossible even in principle. I don't see how a harsher punishment for him would have produced an obviously better outcome. I think it would be more productive to argue that people who commit similar crimes should receive similar punishments as this, rather than arguing that he should have received a harsher punishment.