This is true, however, I think full scale legalization opens the flood gates for trafficking like you said. Porn and strip clubs are already rampantly exploitative in nature. Full scale legalization is like ripping off the scab. I don't think this is an issue people are pushing for unlike say weed legalization. I think at best what the government could do is designate a portion of each state which legalizes prostitution as a "pay for play" zone similar to Nevada. But even then, things would just go underground.
Holland considers banning sex workers aged under 21 and introducing permits for all prostitutes | Daily Mail Online
Even the Netherlands is looking to reform its approach to legal prostitution.
I actually think zones are the best approach. The only reason why I’d argue for a legal approach is because it can be more heavily regulated. I’m not sure if decriminalization alone can help with regulating things more strictly. Any businesses of ill repute (sex/gambling) should be in designated “vice zones” where of age adults can go and partake. The zones for type of activity should kept away from schools or areas unfit for this shyt. Brothels would have to be discreet so they don’t stick out in business areas. Things like street walking is only allowed at certain hours in certain neutral locations. The implementation of these zones should be at the discretion of the state, though I think making it a County by county issue like Nevada is also effective. Plus with a legal industry you can potentially reduce the spread of disease by making testing more widespread.
Like you said, it wouldn’t stop certain things from going underground but at least the underground would be smaller in scope. It can better focus law enforcements efforts imo. Like illegal gambling or unregulated bars are few and are much easier to police because of the abundance of legal, regulated options. If you have a legal, regulated industry of prostitution then the black market would threaten their work and bring them on the side of law enforcement.
Again, I’m really split on this because for all this shyt I’m saying I have lots of nieces and female cousins so I’d prefer something like this not even be an option in society. The issue though is that it’ll always be an option no matter what because of lack of opportunities and resources. So attacking the root causes while decriminalizing the workers (the main victims) and the patrons (most times the victimless criminal) can address multiple issues imo.