The link between drug use and poverty is not relevant. It's only addiction and unhealthy drug use that are problematic. Alcohol being the number 1 problematic drug by far.
You jumped through a lot of hoops to explain that there is an avenue from middle class to poverty through drugs, but any avenue from drug use to poverty pales in comparison to the avenues from poverty to drug addiction. People who have no place in society, because they have nothing worth contributing, or for whatever other reason they end up in poverty, they find good feelings and usefulness in drugs, in the participation with others in a similar position.
The link between poverty and drug use is very very clear.
You jumped through a lot of hoops to explain that there is an avenue from middle class to poverty through drugs, but any avenue from drug use to poverty pales in comparison to the avenues from poverty to drug addiction. People who have no place in society, because they have nothing worth contributing, or for whatever other reason they end up in poverty, they find good feelings and usefulness in drugs, in the participation with others in a similar position.
The link between poverty and drug use is very very clear.