That fails to acknowledge that a) the black market tends to be dramatically less safe b) it drives addicts underground where it's hard to identify and help them and c) the addicts this is meant to help will be disproportionately willing to participate in this new, much worse black market.
Bookies scale very well, a small number of them could serve whatever clientele is still interested.
It'll probably turn out similarly to drugs. Prohibition keeps your average citizen away, and makes the market much worse for anyone left in it.
>b) it drives addicts underground where it's hard to identify and help them
I'm not sure about B, it's pretty easy to identify an addict.
The problem I have with sports gambling is the massive commercialization and advertising. We don't tolerate it for cigarettes, and gambling addictions are much worse than cigarette addictions in terms of financial harm.
Gamblers know where to go, but the purpose of all the advertising is attract new gamblers, preferably the compulsive kind.
Another harm is what it does to the integrity of sports. There are famous cases like the 1919 White Sox and Pete Rose. The people who took part were banned for life for a reason.
Keeping the average citizen away is a dramatic improvement. The pre-legalized-gambling era is in living memory, and we know what it was like.