There's no evidence that shows this is true though, all you're doing is assuming without evidence. For all we know things could remain the same, the problem here is justice. We can't continue building more prisons and wasting more money on policies that don't work.
as for manhunters source I'll let him defend that, I only gave it a quick glance.
If we can not find a policy that does not work, do we simply abandon our efforts and allow drugs to be freely distributed?
Here's another government website's article about the legality of cannibis in the netherlands
Legality in the Netherlands
"The sale of cannabis is technically an offence under the Opium Act"
Only in special cases, under the strictest surveillance, does the government allow cannibis to be used or traded. Under all other circumstances the drug is illegal. Hard drugs, obviously, are all illegal under all circumstances.
It is not a simple unsupported assumption that drugs will spread with its legalization.
1. If there is a substance which is addictive, it will spread as more people become addicted to it.
-We have the obvious definition of addiction to support this.
2. If an addictive substance is wanted by many, more of this substance will be producecd.
-Without limitations made by the law, the supply is free to expand eventually meeting the quantity needed by the people in order to profit... which is constantly increasing due to the "addictive" properties of the substance. We can also base this pattern on the success of drugs in the black market.
Based on our current knowledge of drug addiction and economics, it's safe to say that this is no mere "assumption without evidence".
But I do agree that we need a new policy, but our government must not abandon our efforts and just give up on this matter.