I don't see why we should assume that. Making something annoying to engage in dramatically reduces the amount of people who engage in it. If illegal gambling rings operate, you'd have to 'know a guy' and the gambling ring would -- by definition -- have limited scope.
It's like saying "legalize fent" to protect people who use fentanyl. Like, yea, the problem isn't the addicts, it's that if you can sell the drug in a store, you're going to get 1000x the number of addicts. We need frictional barriers to prevent people from becoming addicted in the first place.
The previous system was fine. We had a couple highly regulated areas where you could travel to (Las Vegas, Reno, a few Indian casinos) for people who were obsessed with gambling. That meant the rest of us were mostly left alone, and not tempted to engage in the vice.