The situation is somewhat like Prohibition, wouldn't you agree? And people are generally agreed that Prohibition *did* cut down alcohol consumption. This article from the NY Times, for example, says
https://www.nytimes.com/1989/10/16/opinion/actually-prohibition-was-a-success.html
30-50% is pretty significant, I'd say. And then I believe it went back up by that same amount after Prohibition was repealed.
Why would marijuana be any different? I'd expect marijuana consumption to go up by about that same factor of 30-50% in states where it becomes fully legal.
Availability isn't the ONLY factor, but it absolutely will be a MAJOR factor.
Yes, I will concede the point that if you artificially reduce demand, in the case of prohibition by the way of the gun, that when that artificial restraint is lifted that usage will increase. But it has way more to do with the fact that there are no criminal sanctions on either the production or use, and the pent up demand finally has a supply to satisfy it. Availability in and of itself doesn't drive demand at all. Or at least I can't remember the time I wanted to do something or buy something just because it was there.
But the initial claim was that increased availability means increased use. I dispute that claim in general, other than the initial uptick in people who were afraid of getting arrested/fired/ostracized finally being able to do so without those consequences. There is plenty of proof of that it doesn't with another popular drug, tobacco. Buying tobacco was just as easy when I was in high school as it is now, and just as available. Yet usage keeps dropping. Part of it may be economics since tobacco costs rise with taxes and such, but I'd wager the big decrease is due to better education about the harm of smoking, and perhaps more importantly, the cultural stigma of tobacco use that has increased in the past 40 or whatever years.
But what I keep going back to is that people who make this claim will never admit that they are going to use. Which is sort of puzzling to me. If increased availability means increased use, how come it doesn't apply to you guys?