Opponents of taxing/regulating marijuana and other drugs frequently maintain that there's just no way we can really cripple the black market. It's true insofar as there's a black market for everything (I saw a lady buy a bootleg DVD from a guy on the subway recently). But people overwhelmingly prefer to do their shopping at actual stores. The burden shouldn’t be on us to prove that pot stores can effectively corner the market on selling pot. Of course they can.
The only reason anyone has a hard time picturing the demise of the traditional drug dealer is because there's so damn many of them, every one of which was created when we stupidly tried to ban drugs.
Anyway, go read this post
from Pete Guither which addresses this point quite well.