Long sentences and reliable enforcement are complimentary.

If you can reliably prosecute the repeat offenders you catch, and put them behind bars for a long time? You stop them from committing more crimes. The crime rate falls, and the amount of enforcement manpower you have available per crime rises. Making it easier to catch and prosecute the remaining offenders.

Most of the low level crime isn't done by "a dumb decision as a teenager or in a lapse of judgement". It's done by someone who has done it 5 times before and will do it again. Unless jailed, that is. The jail doesn't fix whatever's wrong with them, but it is hard to keep doing crimes while behind bars.