Historically speaking, there's not strong evidence of this. Despite bootleggers' existing alcohol production and distribution infrastructure at the end of prohibition in the US, they didn't end up as major players in the liquor market after prohibition was repealed, and it's pretty easy to see why: no sane consumer would choose to buy a product from a gangster in an alley when they could just as easily buy from a reputable liquor store down the street. This was all magnified by the fact that government regulation and corporate oversight ensured safer and higher-quality products at lower prices.

There's no reason to think the same thing wouldn't happen with pot: if you could buy it at the pharmacy, why would you go through a dealer? Maybe if there were huge discrepancies in cost because of high taxes, there might be an opportunity at the margins for some arbitrage, the same way there is for cigarette smuggling in Europe, but I'm skeptical, especially because most of the cost, today, of buying pot isn't paying for direct production costs (materials, fertilizer, electricity, etc.), but rather compensating the various players involved for the risk they assume in producing and distributing the product. This all goes away with legalization.