The marijuana prohibition of the 20th century was a complete anomaly in practically all of human history, driven almost entirely by US policy. Before that humans had used the plant recreationally and commercially without hassle for thousands of years. We'll be just fine.
Unfortunately, it does seem like there is a big uptake in users after legalization, which hopefully will subside.
The things that did the most to drive down use were raising the price of cigarettes (which fund the healthcare costs) and banning smoking indoors, where it affected even non-smoking workers [1]. When we stop subsidizing harmful behavior, we get a lot less of it.
Especially in places that are super-cold.
[1] https://onlinelibrary.wiley.com/doi/pdf/10.1155/2012/961724
So what happened in the ~2000 era window of time? The internet and social media started exploding in popularity. In fact you'll find that the graph of the decline in teen pregnancy [2] looks roughly similar to the decline in young adult smoking, and probably for the exact same reason.
[1] - https://news.gallup.com/poll/509720/cigarette-smoking-rate-s...
[2] - https://www.pewresearch.org/short-reads/2019/08/02/why-is-th...
Technically, of course, vapes aren't cigarettes. But It's a bit misleading to say that smoking is unpopular, when it's just been replaced by a doppelganger. Especially since vapes are also deeply detrimental to health [1].
“Throughout the early to mid-2000s, especially between 2004 and 2007, an increasing number of states enacted a statewide smoking ban of some kind.” https://en.wikipedia.org/wiki/List_of_smoking_bans_in_the_Un...