There's plenty of organized crime arranged around things that should be illegal, for example slavery.

There's no organized slavery without complicit governments.

Bah, there is plenty of slavery without complicit governments.

Typical example: Criminals lure naive poor country girls with prospects of lots of money and a job elsewhere. As soon as they arrive to the destination country, they are held captive (they steal everything they own, and use violence) and then used as prostitutes. I fail to see government help in this scheme.

"The government" is absolutely complicit in that crime because they turn a blind eye to it.

Do you think that the cops don't know about Backpage or other prostitution outlets? Do you think that law enforcement is impervious to corruption?

I don't know about the US, but in other countries, slavery prostitution outlets are indeed persecuted and closed once found. Other prostitution outlets are tolerated (or encouraged.)

My point is even assuming incorruptible law enforcement, these outlets would still exist, might last less if you wish.

From there to saying it's a government sponsored slavery is a big step.

The government is complicit by keeping it illegal.

If it were legal, it could be taxed and regulated like anything else. Most people are OK with letting others do what they want with their own body. It's certain puritanical, tyrannical, authoritarian and powerful government factions that don't allow that.

I'm not sure that by legalizing prostitution the slavery outlets would disappear.

I think it depends on the government. If the taxes or regulatory fees on legal prostitution are too high, it can create a black market similar to that of tobacco or pharmaceutical drugs.