A simple scenario as to why a privatised ATC service will be less safe than a public one is the choice of route. In a public system a decision is made based on what will be the likely impact of traffic further along, in a privatised system it may well be just a case of does our company get the ANS charges from the sector or not. If the answer is no then the aircraft will be put into the busier sector from which our "company" gets the ANS's, safety won't come into it.