OP is wrong.
We want cultural and financial incentives not to do wrong. We also want raw primal fear, if you do this evil shit they’re gonna get you and lock you away forever type fear... if you do this stuff.
And if you accept that that is the goal, it is then logical to put resources toward the most efficient means of accomplishing it, to maximize your chances of success. I find it unlikely that long prison terms are that most efficient means in most cases (an exception might be dangerous likely re-offenders), given considerable evidence that it is ineffective in discouraging most forms of crime, and in some cases can increase likelihood of engagement in crime by ex-convicts.
Edit: there probably is value in some amount of prison time as a deterrent though, as I elaborate on in the comment below. My main point is that I highly doubt there is anything like a linear relationship, if any relationship at all, between the length of prison sentences and the effectiveness of the deterrent. And given that (if in fact it's true,) we might as well keep sentences reasonably short, and use the resources saved more effectively.
Likewise capital punishment does not prevent or lower murderer rate. There is no correlation at all.
So we have to seperate prevention and fear. The fear that you’d have to go to jail is the reason why punishment exists. we want people to know their actions are always held accountable in the perfect system - but unfortunately not.....
Prevention is education and make sure every community has a fair balance of income - a poor community is of course going to have a high crime in theory.
Now, I'm a pretty analytical person, so I'm probably most likely to be affected by deterrents (although perhaps also most likely to believe I could avoid getting caught!) A more impulsive criminal might not consider the possible consequences much at all; certainly not enough to distinguish between a low or high number of years locked up.
So if we take it as a given that long prison terms are not particularly more effective at deterrence than short ones - and I believe there is evidence to back that up, but I'm not an expert; if I were actually in a position to change these things I'd obviously do the research! - and that keeping people in prison costs a large amount of money, it seems to me best to reduce prison terms to the minimum degree possible while maintaining their deterrent benefit, and to redirect the savings into things that evidence suggests are effective in reducing crime (education, social programs, etc.)
Ironically, given my initial comment, this kind of highly calculated crime (the VW emissions) is probably most likely to be deterred by prison sentences. Once again though, I don't expect a long sentence (and 7 years seems long to me for a non-violent crime) would be any more effective than a short one. I mean, would you risk _any_ non-trivial amount of time behind bars just to help your bosses make more money?