It's just how it works. There are diminishing returns to the common drugs of abuse. If given access to high quality drugs of abuse without negative economic and social consequences the body will adapt, use will plateau, and for the majority eventually phase out. A minority will choice to maintain. There are negative health consequences to long term maintenance but it's not like the reality of obtaining poor quaility street drugs in dangerous situtations. It's data driven and based on medical theory so it's not much of a belief. See Swiss and UK medical heroin programs for evidence.