> Have severe jail sentences rid the world of illicit drugs users, murderers, cutpurses, burglars, vandals, fraudsters, rapists or anything else?

No, but you have to put things into context. Prison is a lot worse for a 50+ year old with a few million in assets than someone who's only other alternative is working at a fast food restaurant for minimum wage. When the choice is deal drugs or simply not make enough to survive there really isn't much of a choice.