Composites dramatically reduce assembly time (you just glue the thing together, fewer rivets, etc.), which reduces the cost, composites virtually eliminate corrosion (a BIG problem with older airplanes, just ask the Air Force), and composites have a longer fatigue life, which will allow the airplane to be used longer (assuming its fuel efficiency remains competitive). The latter is a real big deal for a 737 replacement (remember the "convertible" 737 over Hawaii?).
My only question is, with the cost of fuel through the roof, will GE dust off its old "propfan" concept it tested on an MD-80 in the late 1980s. A composite update to the Boeing 7J7 concept might be ideal for the next decade. Especially facing off against a slightly updated Airbus A320.