Making Marijuana legal has nothing to do with making stopping those people that already take it.
Making it illegal is not the same as stopping its use, and likewise making it legal is not changing those people that are already taking it.
My point is that legality and use are not the same.
I also agree with Dave, Alcohol, Tobacco and MJ are sibling drugs. Maybe even the Three Musketeers, all for one and one for all.
To answer the question, I don't know that there would have to be another drug to legalize.