We can replace the X in "X should be legal because criminalizing it will just drive X underground" with (heroin / crack / glue sniffing / child porn / whatever).

Not all of those substitutions work. It seems to be a difficult thing to process, for some people, that outlawing things does reduce their usage, and some of those things are really harmful.

Conversely, at least some of the people who are now gambling on sports really would not have done so, when they still had to know a bookie. No, they wouldn't fuel the Mafia instead; they just wouldn't gamble.