Frankly, I find this entire topic to be so fundamentally infused with modernistic concepts that it annoys me to even broach it, but here goes. (And yes, sure, the setting is a horror-garage-pastiche. Okay. Still going to point out where it doesn't make sense.)
Would average Barovians even be able to afford opium? Opium was not an everyman's drug
[1] until it was mass produced in India by the British (who then went on to wage a few wars to force China to open their markets to this cheap opium; not our most edifying moment in history). The point is that in China, where it was consumed as a narcotic and smoked, it didn't become a problem until it was cheap -- and it didn't become cheap until the British brought post-Renaissance entrepreneurialism and modern farming techniques into the picture. In short, it wasn't a major concern for the Chinese authorities because until then the average peasant simply couldn't
afford the stuff. It was a pass time of bored nobles, and if a few bored nobles decided to slowly kill themselves with opium in Barovia, I think Strahd would see that as a tidy way of keeping them from plotting behind his back...
In summary, why is opium even a problem that is deemed addressing in most cases? Is the Hazlani opium trade really that aggressive?
I mean, if Strahd is that puritanical, he ought to be banning things like wine and tsuika -- I am sure those have a far greater impact on his peasants' productivity than expensive exotic drugs.