You are assuming prostitution is driven by demand-side concerns alone (or that eliminating trafficking gangs has no effect on the supply side.) Trafficking gangs using addiction to trap people into prostitution is not unknown, so eliminating criminal trafficking gangs should have supply-side impacts as well.
I was thinking about the angle "the drug trade vanishes, so the gangs will drift into pimping" - which is, imho, not really valid since where prostitution is illegal, the gangs are already in pimping and the supply/demand is more or less matched, so there are no new profits to be made there.
