Did you suddenly get an epiphany and realize what we are talking about here or are you being facetious? This is what everyone has been saying to you all along, and this is the critical difference between legalization and decriminalization.
Which btw.. it seems you do not know the difference between the 2. Reading through this thread I have seen you misuse both terms, or treat them like they are synonyms.. they absolutely are not.
Decriminalization is merely making it akin to a parking ticket for minor possession, it still leaves the black market intact, and in fact encourages it to thrive since it is still illegal to distribute drugs - this is not a solution, and it also is not what the subject of this thread is. Decrim is a concession, and a half measure that addresses a few minor problems, while ignoring the major ones.
Decrims only practicality is that it is all that can be done currently due to international treaty (Single Convention on Narcotic Drugs, and its sister treaties), and is a necessary baby step that will take off a few pressures, and show that a change in policy will not equate to the sky falling, it is a necessary (albeit flawed) stepping stone to ultimately changing opinion, and drug policy globally.
legalization is another animal though, this is where the scenario you mention above comes into play, legitimate business interests can now get involved in legal (yet regulated) distribution.. and yes it will starve the black market until it is a mere shadow of its former self.