I do agree that magic as drug addiction is still the biggest misstep in the season...but even that one I've softened on a bit even if I do consider it a botched execution. You say its previous thematic role in the series is hurt by it, but I would argue that the overlap between magic/occultism and drugs was thematically kinda a thing going back to at least season 2 if not earlier. See: Giles past as ripper and the summoning of Eyghon, or the candy that turned everyone into teenagers. Magic as a factor in abusive relationships is also a pretty consistent theme also since season one such as with Amy's mom.

I would argue the biggest problem with the metaphor in season 6 is that they went too far and made it too literal to the point it feels incompatible with the mechanics of the world up to and after that point. I think there actually was an interesting story to tell in the way Willow became dependent on magic to handle all of her problems and the way she used it against the consent of Tara to solve relationship issues. The magical crack-house though....I don't know who came up with that but that was a bit too on the nose and head scratching. I guess since it was the early 00s they were worried people wouldn't understand the issue without that or something.