You can look at other societies where they spend more time rehabilitating offenders (e.g. Norway), and Norway does have lower crime rates, but which way does causation run? You can argue that rehabilitation lowers crime rates, or you can argue that low crime rates makes the society tolerant of relatively lenient punishment.

And if it turns out that retributive punishment does work effectively, are you willing to pay for increased crime rates just to find out that our current system is alright?

This type of what-if paralysis isn't rationally supportable. Some similar examples to show where the logic fails:

And if particle accelerators do produce black holes, are you willing to destroy the planet just to prove it?

And if it turns out Cthulhu does live on the moon, are you willing to unleash its wrath by visiting?

Instead of paranoid fear of worst-case hypotheticals, we should be using empirical evidence to guide decisions. We can look at other countries. We can look at differences between states and counties. We can conduct experiments in smaller areas and roll them out over time if they work. Or we can just have some basic human empathy.

Its funny how the goal that I wanted to work towards changed from that game to a successful life.