That is maybe, because most of them start doing hard drugs, when they are already not functioning well. And then Heroin gives them the rest and they turn into messed up junkies and criminals.

And since it is illegal, it is hard to get data on how many people use hard drugs while maintaining a normal life. Those who are successful, don't draw attention. I always thought otherwise, but was surprised a couple of times, by finding out seemingly normal people use heroin on a semi regular basis. So apparently some people can do it.