Locked hardware really has nothing to do with enforcing a fixed-term contract. I think the day will come when Apple refuses to produce locked iPhones and sells only unlocked devices to the carriers.
In many markets the carriers already do next to nothing to promote or sell the iPhone, so there isn't much they can threaten to take away. And a carrier isn't going to drop the iPhone as long as there's a competing carrier that's willing to sell the iPhone.
The carriers don't have to promote any specific phone. The devices they invariably do push are the ones that make them the most money per subscriber per month, and contain features not found on competing carriers.
In many markets the carriers already do next to nothing to promote or sell the iPhone, so there isn't much they can threaten to take away. And a carrier isn't going to drop the iPhone as long as there's a competing carrier that's willing to sell the iPhone.