I am mainly speaking about the friction that exists for the accessibility of the vice, and that you generally want friction for things that are detrimental to the individuals and society.
For example - in the US you have to be 21 to legally consume alcohol. Does that mean that nobody under 21 drinks? Of course not. But it certainly means that the consumption in that demographic is less than it would have been otherwise, since kids can't just roll up to the supermarket to get it, and since they worry about some (non-jail-level) consequences like getting in trouble with the high school dean or the college dorm director, plus the friction of not being able to consume it too publicly.
Legalization has removed the friction from pot. An average individual smokes more now that they don't have to work through a "connection" to get it - so more accessible - and they can now do it out in the open vs discretely. So the consumption's gotta be way up.
Re: your point about jail. I don't know about other locations but in NYC, nobody has gone to jail for pot in a long time unless they were dealing massive amounts. I think that was a better model - it kept the friction higher for use while still enabling folks to consume or sell a small amount as long as they were discrete. That's gone now.