That's ridiculous. They sell drugs because it is the Easiest way they know. Without drugs, they'd need to find some other, less easy way. Supply+Demand teaches that those other opportunities would be less available
They could go into other illegal fields, or they could move somewhere where their illegal drug expertise is useful, or they could become farmers or teachers or simply retire.
There are other countries and other states in the USA that have legalized certain drugs, have they had a comparative rise in other illegal and dangerous activity like you have claimed?
Or are you just hypothesizing one possible outcome that has no evidence or bearing on reality, "whatsoever".
They sell drugs because it makes money. Legalize drugs and they'll make money some other illegal and dangerous way.