That would be dependent on your version of legalization. Currently, none of the states that have legalized pot have a system in place that prevents cartels from being part of the picture. This is why I believe it is being used to launder their money from other industries (like heroin and human trafficking). It would match what happened with the mafia after the repeal of Prohibition.
The more addictive a substance is, the more of a problem it becomes for society. Heroin, meth, and cocaine are far more addictive than pot and alcohol. Now, you could argue that nicotine is on par with heroin, but the difference is that nicotine doesn't usually kill people. The most destructive aspects of tobacco come from other components of the smoke. The nicotine is just the active ingredient that causes addiction. That being said, I actually would be ok with banning nicotine.
I agree with your premise on welfare, but we're not there yet. Until we are, other policies have to take this into account. It's why the open borders position that many libertarians often support isn't feasible.