As it seems to me, who hung around with a lot of drug users back in the day, as well as regular folks: most people who are interested in trying them can and will get their hands on it regardless of legality, sometimes easily. It's about as low risk of a crime as there is. Those who aren't interested, won't, again regardless of legality. There will be edge cases where somebody will go "Ah what the hell, it's legal now, why not" and toddle on over to their local dispensary for the first time but largely speaking anybody that wants to smoke weed or snort coke is probably already doing it.

Now what probably would change is the number of people on record using drugs, per capita, over the next few generations if it becomes normalized like alcohol has been. Which makes sense. But, counterpoint to that, in countries where they have legalized many drugs they still often have lower rates of severe addiction because they've generally also set up safety nets for those folks. Accessible medical care and available addiction treatment options will keep many drug users from hitting rock bottom, but we don't really have that in the US so many users will often go unassisted in any way for ages and lose jobs and homes because of it, only getting "help" when it becomes forced upon them by the state (which is frequently not in any way helpful).

Anyway, I'm rambling, but tl;dr it's definitely a multifaceted situation and blanket legalization probably isn't a great move without accompanying medical and social support, which needs to happen anyway regardless of any moves for drug legalization. Gotta walk before we can run, unfortunately.