If someone did want to do a study to isolate the effect of previous prosecution on future criminality, you could look at the rate of future criminality among a population of offenders who were not prosecuted, ie people who got away with it. Additionally, we can discuss well established socioeconomic causes of criminality and how past criminal convictions can increase one's likelihood to experience those socioeconomic conditions associated with criminality. There are some situations in which "that argument is pretty reasonable, we don't need to spend a generation on a longitudinal study to establish causation while we continue to ruin people's lives with counter productive punishments" is the correct stance to take. Criminal justice reform for non-violent offenses is one of them, in my opinion.

Beyond that, prosecutors are probably on average pretty average people, so why should we expect them to somehow be masters of character judgment? Surely there are some star prosecutors who are, but those few aren't prosecuting every or even most cases. Most cases don't even get to actual prosecution because people are pressured into plea deals, so to imagine any actual character judgment is at play there is a misunderstanding of how our criminal justice system functions.