I suspect if it was legal, a lot of people would move towards tobacco-substitutes to mix in their joint rather than actual tobacco. Amsterdam coffeeshops already sell pre-rolleds with "mix" in them (since smoking tobacco is illegal in there), I'm a cigarette smoker myself and I have no issues replacing tobacco with "mix" when I'm over there.

(But right now for typical weed smokers: "mix" isn't easily found to buy, while tobacco is always easily on hand. If it was sold in legal shops they could offer mix at the same time, something there's no incentive for illegal dealers to do.)