I think that with fuel costs being considerably higher, engine efficiency and lighter weight airframes are becoming a strong selling point for future sales.
The is nothing wrong with the 737, but that doesn't mean Boeing can't do even better with newer technologies, and since a completely new aircraft takes years to design, test, and ramp up a production site, it's wise for Boeing to be looking at their options now.
They may discover that simply upgrading the 737 again is the best route.
Thanks for the excellent points. It is wise to always be exploring options. My point is that the 737 fits perfectly for the markets they are serving and the other segments (smaller jets) are also adequately served by other manufacturers. There may be an opportunity for Boeing but there is no need going unmet.
I don't consider more efficient engines and other modifications to be anything that could be called a new design.
However, you do make good points.