Example 20 year old experimenting with drugs:

Going to jail for a year breaks any career chances or most of the job opportunities plus messes up his mind by staying with other convicts.

Letting him experiment with drugs, he might mess up his health but also he has still a chance to continue rather normal life.

If he manages to get out. Gets addicted to meth, can’t work, needs more meth, what does he do?

What does a 20 year old alcoholic do? What does a 20 year old gambling addict do? It's wild that people even think of prison as an acceptable answer when there are so many analogs of things that people can abuse or get addicted to, things which we don't criminalize

Thing is - you don't need to show the likely outcomes are perfect - you just need to show they are better than what happens now.

And right now it seems to be "if you do this thing that sometimes has a really bad outcome and we catch you then we'll make that outcome much worse"

One point people don't tend to know, is that a lot of folks actually get drugs in jail, and often prefer them. There's quite a few opioid replacements that get offered to anyone who can show addiction withdrawl, and many folks say they're actually a better, longer high than the street stuff.

There's also some revolving door, and 'Shawshank' style issues, where folks rotate out for a couple months in the spring / summer, do whatever on the street, and then rotate back in the fall / winter with some dumb crime. Eat, rest, stay warm, get the opioid replacements, then head back out. Kind of a homeless shelter where you just have to do some 3-month misdemeanor stint to get room / board.

Although long incarceration can definitely be an issue, there are also some folks who've made it a lifestyle.