Not finish, or renegotiate? To not finish your contract costs a penalty. A customer who renegotiates their contract, even after only a year, is a 'lifetime' customer. Take a customer who has been on their contract for one of the two years, and renew them for two more, and you have the equivalent of a three year contract. They get their money one way or the other... and I don't mean to say that someone in a contract should necessarily get the introductory price, but certainly a nice reduction.
I'm being a devil's advocate of sorts here... because I AM on a two year contract, and I WILL fulfill it, because I don't NEED a new phone every year, but I am amazed at the amount of consumer 'sheep' there are here. The consumer/retailer relationship has historically been one of offer and counter offer. We've become so accustomed to seeing a price, and settling for it in most areas of retail sales, that bargaining has all but gone out the window. If we as consumers don't push the envelope a bit, we're doing ourselves a grave disservice.