Torque
Not all carriers are weight limited; some are volume limited and the 744 still offers more room. Furthermore, a factory fresh 744 freighter comes with a nose-door, which CR2 will tell you is the best thing since sliced bread for loading loooooong pieces.
I would venture the estimate that a converted 744SF will be cheaper than a brand new 777F, which may further justify its continued existance. List price for a 777 in pax configuration is in the 150-170 million USD range; a freighter would probably be a tad more expensive (as the 744F is a tad more expensive than a pax 744).
LHC have indeed expressed an interest in the 777F, and I suppose it is not outside the realms of possibility to see LHC as a launch costumer. They may add the 777s to increase capacity, or they may ditch the MD11s and go pure 777, or they may get 777s to replace the 742s transferrred to and operated by AAI.
Emirates have also expressed an interest in the 777F, to feed their A380 Freighters. UPS would probably also be a prospect, to replace their aging 747 classics, and there are bound to be many more operators that could replace their classic whales too.