The deterrent has to be enough of a deterrent to actually deter people, else it's a negligible downside risk (like a slap on the wrist fine and job loss for this case). Yes, execution would certainly put a stop to this behavior, even more so than jail time (and could also help with prison overcrowding), but I think to err is human and execution is a little too extreme and would cause society to be unhappy and scared.
Yes, I think there are a lot of extreme prison sentences in this country and we incarcerate way too many people. But I think it is ridiculous to be so lopsided with punishments and only put people without means in jail. White collar crimes like this one have a larger negative impact on society than some guy caught stealing a car and they should not get off with a slap on the wrist.
I don't think prison is for rehabilitation. Some humans act incredibly selfishly and if there are no consequences to bad actions, they will act selfishly and they will have a negative impact on society. Law enforcement and criminal justice is a balancing act with no perfect right answer. It is not tenable to be too far on either end of the spectrum and I think we need to try to strike that balance where crime is deterred enough but where we don't live in a police state dystopia that causes a depressed and miserable society. I don't really know where that point is, but I am all for equitable punishment of white collar criminals.