Even if marijuana were legal, I doubt that would happen. Beer, wine, and hard liquor are all currently legal, yet people usually prefer one of the three.
For example, I've seen few beer drinkers who will "switch" to wine or hard liquor as their preferred drink. I just can't see people who prefer alcohol switching to marijuana.
Your conclusion does not follow from your premise. That few people switch from one kind of puke-and-hangover juice to another says nothing about whether people would switch from puke-and-hangover juice to marijuana.