In the Netherlands, where soft drugs are tolerated (not legal, but tolerated), almost all customers of the so called coffee shops are people from neighboring countries. Plus, the laxism towards drugs in general in this country make it a perfect place to produce and import hard drugs that will provide for all the networks in Europe (the virtual suppression of all European borders not being of great help to prevent this).
Another question would be if people would still "enjoy" their marijuana if it came from big corporations that would mass produce it.
I also don't believe that the state would gain any kind of substantial benefit out of that. It's peanuts compared to all other forms of taxation.
I believe that legalization will simply shift the trend onto harder drugs. Cocaine has become pretty much mainstream in the last years, so did synthetic drugs. When the consumption of these will become out of control, just like for marijuana now, you would have no choice but legalize those too.