I don't think we would ever legalize cigarettes and alcohol if they weren't already widespread. You cited the numbers yourself, it would be a terrible decision. But lots of people are already using these substances, and we've seen how well it works to make alcohol illegal... The reason cigs/alcohol are legal has nothing to do with their addictiveness or the harm they do, so a comparison with them.

The anti-legalization thought is simply: Cigarettes and alcohol are already bad enough, let's not add a third substance!

(Note: Personally, I actually support weed legalisation, because it eliminates black markets and other problems that illegality brings. But that's a whole other, more nuanced argument. See the frontpage thread about steelmanning...)