[Know your rights]

I simply acknowledged that factors other than illegality affected drug usage, either in a positive or negative direction. I never discarded illegality as the reason for the decline.

You should have, since once you acknowledge that factors other than illegality affected drug usage, your argument by correlation-thus-causality no longer has any foundation.

[robertpaulsen]

If illegality did not cause a decline in usage, then we can conclude that there wouldn't be an increase in usage if the product became legal?

Surely you're not asking me to believe that?