So the problem is that someone who does one crime is a criminal and being a criminal means someone does every crime? For example, someone who removes DRM from a video game is a criminal, and therefore he also shoots people, so we have to put DRM removers in jail to decrease shooting deaths?
I have to admit, I'm not really following the logic there.
Because you made a strawman out of their point. Reasonable people avoid drug markets for good reason.
Drug crimes and non-drug crimes (assault, robbery, x/y/z) commonly cluster[1], and citizens with working risk estimation skills move on and cede the space.
[1] https://pmc.ncbi.nlm.nih.gov/articles/PMC2719901/