One of the justifications for mandatory minimum penalties is the incapacitation principle, or denunciation. We don't have evidence that shows that MMPs have a deterrent effect, but they clearly have an incapacitation effect, or a denunciation effect by society.
Yes, if a person is locked up, then they're not on the street. But MMPs do not have the same effect with respect to drug trafficking. When you pull a drug trafficker off the street, there's an economic incentive there and the void is filled. As long as there's a market for someone who wants to buy drugs, if you take a person off the street there'll be a new person coming along to push the drugs.
The effect that penalties have depends on the crime.