“And this qualitative improvement happened while pot was illegal.”
Side note...it makes sense that cross-breeding and optimum horticulture would result in qualitatively better pot - but from the subjective standpoint, I’ve never heard anyone who’s tried both say that the current stuff is even close to 20x as “strong” as quality pot in the 60’s. Reefer madness?
The grow shops do make some crazy potent stuff. 20X more potent? I doubt it. Even so, more potent means fewer tokes for the desired effect. This is a scare tactic used by drug warriors.