Prohibition didn’t work after alcohol had already been legal. There’s no comparison. In every state that has medical marijuana available we see increases in underage use.
Both alcohol and tobacco are becoming increasingly restricted by law. We can no longer smoke in our offices (or our schools). We cannot drink over the legal limit (which continues to decrease) and drive. By prohibiting or limiting use with both of these substances we’ve seen appreciable reductions in use in the general population.
Look at alcohol, it currently continues to be the drug of choice among teenagers. It is available legally. How is that not going to change if we legalize marijuana?