Not to mention the fact that the populace has been ingesting intoxicants pretty steadily up until now as well. The only difference is that it's mostly been alcohol.
The idea that you can be jailed for unhealthy behavior (when it doesn't affect other people) is relatively new I think. If anything, I feel like liberalizing drug policy would be more of a correction than a radical departure from history.
For as long as I'm aware, some portion of the population will desire recreational drugs, whether that's alcohol, caffeine, nicotine, cannabis, or whatever. A smaller subset will run into issues with their use (whether that's for physiological reasons like genetic disposition or because of life circumstances that make intoxication seem like a more appealing escape).
That won't really change if you can no longer be put in jail for ingesting something. I'd imagine it would be a lot like it is now. A good chunk of the population would continue to chemically alter their moods and mental processes on occasion and a smaller subset of them would run into dependence or other health issues.
The difference would be that people wouldn't be at risk of prison or permanent financial ruin because they did something that is (at worst) unhealthy or unwise. Intoxication would and should remain no excuse for otherwise criminal acts committed while under the influence. You're still responsible for your actions, you just wouldn't automatically become a criminal because you adjusted the way your brain works for a few hours one afternoon.