This is a great plan for a country with limited government resources for law enforcement, prosecution, and imprisonment.
Who would you rather have rotting in any particular Mexican jail cell, a Mexican who is a violent criminal and will serve his full term. Or a Mexican violent criminal who was released early and jumps the border to the US, because the cell is needed for 2 non violent druggies to do their terms.
Who would you rather the local police demand bribes from? The local cokehead or local drug smuggling rings?
How many officers would you want in narcotics division as compared to beat cops?
How many docket slots would you want filled with petty drug possession cases, as compared to serious criminal trials?
How many underclass young men of mestizo background do you want to incarcerate for petty drug crimes at a net loss to MExican society, when if you leave them be they will probably run out of options in Mexico and head across the border and become net positives through wage remittance to family members in their home communities.
I'd rather build a huge wall, ship all the illegals back to Mexico (both free and incarcerated), and seal the border up so tight an ant couldn't even get through,and force Mexico to simply deal with it themselves. The druggies are their people, not ours.