Prohibition has generally been a lot less effective than regulation. The problem in the US, in my view, is much more the utterly-gutted effectiveness of the regulatory state than it is the existence of legal gambling.
Who says we need to keep going? That's not a hypothetical question - who says? Why would we do that?
I agree regulation is good, but prohibition is a type of regulation. There're also levels of prohibition - you don't need to prohibit all of it, maybe just the most obviously harmful.
Like you can ban online gambling but keep casinos if you want. I don't know, I don't have the analysis on which is worse.
Why wouldn't we? If we're legally precluding people from hobbies that could harm them, there's always going to be a worst legal one.
We literally have a system of tiers of addiction versus potential value for addictive substances. A slippery slope argument is pretty silly when we literally already created a staircase.