But doesn't any such successful agency become a state in its own right?
A polycentric legal order would have people living in one area who might subscribe to any number of different private security and arbitration firms.
Perhaps you could say their different agencies come to some sort of an agreement in regards to disputes or conflicts between the laws subscribed to by different people. But where such agreements exist they would be made for the mutual benefit of the corporations and not necessarily their customers. To me the end result still looks a lot like a state, just one in which cartels stand in for governments.