Just declaring something legal or illegal is not sufficient to solve any kind of societal issue. The right laws need to be in place to be effective.

I have no idea what Germany did, but if all they did was one day say "hey, it's legal to be a sex worker now, go nuts!" ... then the outcome they got is not surprising at all. Even if they did more than that, it's a complex issue, and you need a comprehensive legal framework in place in order to ensure that exploited sex workers have a path to justice.

If the response from exploiters is "well, sex work is legal", and the law buys that and looks the other way, then the law was not made particularly well.