The facts on the ground are that, dissuasion or not, these economies will never go away ("oldest profession", after all). Full legalisation would allow for complete oversight of such murky sectors, which would ensure everyone's safety better than the current arrangements, at all levels. For example, in many countries there is no way for prostitutes to legally ensure their own security, because any sort of professional relationship with a sex-worker is illegal (even if the business itself is not); at that point, whether they have recourse post-abuse or not, is basically irrelevant.

The legal situation around cannabis and prostitution is just a shitshow in most places, driven as it is by outdated sensibilities which have ossified through short-term political calculation. There is no point trying to find in it a logic that is simply not there.