I am not concerned, I think its naive to assume it would not happen. Which I think most people think it won't.
Regarding cost of violence, yes, its not cost effective in many cases, but you overestimate the cost of violence. Assume someone is behind in paying you $50,000. It would help payments to send 3 armed men to their house once a week for a whipping every week they don't pay and charge them $500 fee to cover the service as part of the contract. I guarantee you would collect faster and I guarantee people would sign it (in order to borrow $50,000 to maybe start a business or help buy a house).
Also, why is pimping make financial sense to both the pimp and the chattel? Has little to do with the legality of the industry. Make it legal, there will be pimps too (see all of history).
I never thought this way until I met more types of people in the world. Most people live in a bubble, I used to. People will sell their own life if you give them the opportunity. If you really don't think so, you need to get out more.