That's an interesting take but I think it's mostly reasoned from flawed first principles, as if everyone is a rational actor. For starters, some people do meth just because their friends are doing it. Some people aren't able to see the consequences clearly.

And even if you assume it's only people having a bad go at life, every life includes bad parts, despair, etc. We're all vulnerable to irrational acts in those times.

Legalizing drugs just makes access a little bit easier during those times. Once they're addicted, though, no rational amount of jail time will dissuade anybody.