Except who needs a 737 composite replacement when the Next Gen is going so strong? Maybe in the year 2020!
If the manufacturing costs of the composite 787 are really significantly lower than for conventional aluminum, Boeing would be in a position to significantly dent Airbus's share of the narrow body market. Sure the 737 line is doing well, but the narrow body market is so large (Boeing builds almost 30 737's per month) there could be room for more than just two manufacturers. Embraer has already launched the E190 which is larger than a regional jet. Canadair is working on an MD-80 sized jet that could compete with part of the 737 line especially the 737-600 which is a bit heavy. Replacing the 737 may be like eating their own children. But hey if Boeing doesn't do it, someone else will.