Human sentiments are the the primary driver of human behavior. You get better results out of harnessing those sentiments than by trying to treat people as purely rational machines.
Maybe, maybe not. But the point is that the system should leverage people's sentiments for the sake of minimizing crime, and not be designed the way human sentiment would want it to be designed (in this case, yours).
If retribution turns out not to minimize crime, then there's no place for it in the justice system. So the only question becomes whether or not retribution minimizes crime more than non-retribution methods.
It’s not really possible to measure this over the long run. Short and medium term studies show that mass incarceration dramatically reduced crime in the US, mostly by literally removing violent young men from society, but that’s not the main reason why I support strong punishment. The moral decay of a society can take a hundred years to set in, but once the consequences become apparent, it can be too late to do anything about it. There is a lot of wisdom encoded in the ways of the past. Retribution for crimes is a universal institution for a reason.
There's also a lot of bullshit encoded in the ways of the past. Just because you like one of them doesn't make it good for the society.
And talking about "moral decay" (as if morality was some objective set of rules that society can either follow or not follow, and not a huge continuum of various moral standards of both individuals and societies) makes me quite sure that you're speaking from sentiment, rather than from reason.
"It feels right to me, therefore it is right" is not a good argument for convincing anyone of anything.
I agree that it's very hard to measure this sort of thing. There are way too many (constantly changing) factors that affect the crime rate, that it's almost impossible to isolate a single cause and measure its effect.
However, that should only motivate us to approach the problem even more rigorously, rather than to resort to subjective heuristics.
