I’m going to argue that those who are prone to use them already are. They’ll just have less risk of being robbed when buying them, and be more sure of the quality/safety.
I also think the only people discouraged from buying drugs due to current laws are middle aged, middle class folks who can’t afford to risk a citation or arrest for buying pot due to their occupation. Teens and 20-somethings use fairly commonly, and older folks get their medical cards if they want it. I couldn’t risk buying anything due to career implications. Decriminalized status helps me not at all, even if I wanted to just stay in on a Friday night and cause no one any harm.