By legalizing it, you effectively add cost, or tax, and the illegal component continues to remain (unlesss you simply make it stop being illegal, and unregulated, which most people agree is a bad idea) and the market segregates the workers into legal and illegal workers. The illegal workers lives get even worse, and the legal workers end up losing a lot of their money.

Look at legal marijuana, or hell, even legal prostitution in places that allow it and you'll see that's been the real effect.

but that's the point, legalizing or decriminalizing the actual act enables the people involved to seek protection from the illegal components.

Look at legal marijuana, or hell, even legal prostitution in places that allow it and you'll see that's been the real effect

at least in germany the opposite effect seems to be the case:

https://en.wikipedia.org/wiki/Prostitution_Act

"Sexual acts in the context of a prostitution contract are now valid to establish claims for compensation since the law came into force. This has not only significance for civil law, but also criminal law (property offences). In addition, prostitutes can now regularly insure in the statutory health, unemployment and pension insurance."

https://en.wikipedia.org/wiki/Prostitutes_Protection_Act

"effective protection for prostitutes is only possible if this work is legalised"

in addition to that, the issue i see is, how much control is necessary to enforce a ban on prostitution. i would argue that it is not possible without a serious reduction of personal freedom. legalizing it enables to focus the control on those engaged in such activities whereas the rest of us won't be bothered.