I think the big impact will be sales through acquaintance/friend networks though, this will make it much easier to support an addiction through small sales.
There'll be more money flowing to the traffickers so they're not going anywhere.
I don't exactly think people actively weigh the probability of punishment when considering to consume drugs. There's a wealth of research that suggests stiffer penalties don't dissuade drug use. If this is true, then the inverse is likely true.
I also think it's fairly well understood that traffickers thrive best in an illegal marketplace, since the threat of violence and imprisonment is effectively a barrier to entry for people with less violent means. If the outcomes of marijuana legalization hold true for other drugs, we'll see more market participation from legal organizations and that additional market competition will drive the violent cartels into other unregulated markets where their competitive advantage in violence can be better leveraged. The takeaway is that violent cartels work best when they can exploit their advantage in violence. Take away that advantage by making the market legal, and you cut the economic legs out from those organizations.
But it will also help people who want to seek help because now it means the first step doesn't include admitting to a crime.