The basic problem here is that, like the drugs, there is a huge surplus and dealers are replaced as easily as a check out clerk in a super market. Too often it is teens who fill the position.
Putting a dealer in prison creates two dealers, one in prison at our expense who will eventually get out, and the person[s] who replace that dealer.
"... suppliers have an incentive to substitute youth for adults in the distribution chain. ... the unintended consequences of the rising enforcement are that more people are engaged in supplying a smaller amount of drugs and more juveniles have been lured into the drug trade."
David W. Rasmussen : Professor of Economics and Director of the DeVoe L. Moore Center at Florida State University.
Bruce L. Benson: Distinguished Research Professor of Economics at Florida State University. (Ph.D., Texas A&M University, 1978)
Note: The reference to "a smaller amount of drugs" is due to the fact that to evade seizure, suppliers make the drugs purer and more compact and not due to lower consumption. This tendency for illegal drugs to become more potent - the reverse of our experience with legal drugs - has been called "The Iron Law of Prohibition."