Seriously? The underground drug market in Canada is smaller? Where are you getting your numbers? The fact that the taxation of weed is so high the underground economy in that particular commodity has not really changed that much.
https://qz.com/1605614/canadas-black-market-for-weed-is-thriving-even-after-legalization/
https://www.cbc.ca/news/canada/new-brunswick/cannabis-one-year-illegal-sales-nb-1.5323130
Any shortfall has been replaced with the relatively recent introduction of meth and those sales and users are increasing. Drug dealers give out meth to increase customers. Meth is very addictive.
https://newsinteractives.cbc.ca/longform/crystal-meth-addiction-canada
Besides - you are comparing the entirety of a Canada with the specific drug problems and violence of many inner core US cities. That is so wrong on so many levels I don't have the time or space to deal with such an incredibly huge lack of understanding and knowledge of the situation.
As an example of inner city drug use - because that is what Dr. Brantingham was talking about - have you ever spent time in Canada's own Downtown Eastside (DTES) of Vancouver? $350 million a year goes into drug use amelioration in that drug trade community alone and that area has been basically off limits to the Vancouver police for years. The economy is drug sales and welfare and wannabee saviours getting paid to run programs that have shown no effectiveness in decades.
Even the hookers (except the most desperate ones) have moved out as very few people with money will venture into that area.
The drug use in DTES has not been falling, it's been rising. The numbers of deaths from opioid addiction has been horrendous for a couple of years due to the introduction of Fentanyl and carFentanil and yet the numbers are still rising!
So far - the illegal drug trade has not disappeared or decreased in Canada and the human suffering because of it has increased so using Canada as an example of good drug policy is ridiculous.
No-one is "claiming that maintaining an underground/black market economy is a valid reason to keep drugs illegal" as you put it. Dr. Brantingham was calling for the legalization of street drugs back in the 1970s (when this took place) and I am in full agreement with him.
However, the question is still valid: IF you remove the drug trade - what are you going to replace the money it generates with? I suggested the money that is used on enforcement could be used to subsidize the creation of jobs. There will also be money for addiction centres - hopefully. However, as we see - throwing money at addiction and a hands off attitude by the police (basically decriminalizing both soft and hard drugs) has not stopped addiction or the corresponding black market sales (dealers will always find something to deal) in the DTES.
The experiment continues. No-one has the answers - yet.