Whenever someone quotes this, it is very easy to refute, using the DEAs own numbers.

Bare minimum amount of THC to get high is 3% (which is atypical - even in the 70s, I was there!). This would be weaker than the worst (non hemp) ditch weed you ever smoked.

Average street potency is around 8% with premium weed averaging around 13%.

20 * 3% = 60% THC or just about the best hash available. The ABSOLUTE BEST cannabis available may reach 26-28%.

So even granted their false base assumption, the math is totally off.

What actually has happened, is that people generally have become connoisseurs, ONLY choosing better grades of marijuana.

I have smoked stuff 30 years ago that was as good as anything today, but was harder to get a hold of.