So, despite your facile “Sigh,” whether abusees are helped or not by criminalization might depend (among other things) on the details of how such laws affect the signal-to-noise ratio of how vulnerable populations appear to the outside world.

Making (keeping) prostitution illegal will harm some by a black-market effect, and legalizing it will harm others by increasing the core of a practice whose thus-increased margins will always be ugly. Do you claim to have an analysis comparing the two? To your “sigh,” I say, “ugh.”

I changed my mind after looking at the experiences of other countries. The Netherlands formally adopted the legalization model in 2000, and there were modest public health benefits for the licensed prostitutes. But legalization nurtured a large sex industry and criminal gangs that trafficked underage girls, and so trafficking, violence and child prostitution flourished rather than dying out.
As a result, the Netherlands is now backtracking on its legalization model by closing some brothels, and other countries, like Bulgaria, are backing away from that approach.
In contrast, Sweden experimented in 1999 with a radically different approach that many now regard as much more successful: it decriminalized the sale of sex but made it a crime to buy sex. In effect, the policy was to arrest customers, but not the prostitutes.
Some Swedish prostitutes have complained that the policy reduced demand and thus lowered prices, while forcing sex work underground. But the evidence is strong that the new approach reduced trafficking in Sweden, and opinion polls show that Swedes regard the experiment as a considerable success. And the bottom line is that if you want to rape a 13-year-old girl imported from Eastern Europe, you’ll have a much easier time in Amsterdam than in Stockholm.