Going to try come back and answer this one in more detail later, but the gist is that while legalisation does increase trafficking, it's not the prostitution at fault, but the fact that countries have different wage/living standards and legal migration is really difficult. People give money to traffickers to circumvent that, the money then goes on to funding guns and slavery. If you want to stop the bad bits you need to make migration easy and/or remove the incentive to migrate by improving living standards and fulfillment, which starves the traffickers of income and makes them more vulnerable as the coerced individuals aren't going to be penalised by the state for coming forward.