In the 'purely rational' crimes, it's going to be some sort of function of both right? The expected benefit, the risk that you get caught, and the severity of the punishment.

While I agree that people conducting corporate fraud think they will get away with it - I don't agree that the long sentences won't act as a deterrence. If you set the sentence for these sorts of crimes to 1 year rather than 15+, that completely changes the risk profile for people who think there is a 90% chance they will get away with it.