Just speculating, but it seems that the number of people using hard drugs wouldn’t really change. Trends might bounce around a bit after a legal shift, but the people who are more likely to use would still likely use.
Changing the policies around how people are treated as users is an awesome thing! However, money would still need to be pumped into prevention education and anti-trafficking enforcement to start reducing availability and use. (Anti-trafficking is a double-edged sword, and prevention education effectiveness is subject to massive debate.)
My point is that if hard drugs are decriminalized, it probably wouldn’t change an existing trend.