Nice conversation but ... if it ain't broke .....
Aircraft aren't cars. They are maintained according to schedules based on hours of operation not based on parts malfunctioning.
It would be stupid for Boeing not to conduct studies now about a replacement for the 737 in a decade or so. If Boeing doesn't build a 737/A320 replacement, someone else will. Just look what happened to McDonnell Douglas with their DC-9/MD-80/MD-90 aircraft or their DC-10/MD-11.
It ain't broke, it's just outdated. The 737 will have to be replaced with a composites based aircraft. The immense success of the Dreamliner is proving that. The reduction in the complexity of building the fuselage will also mean more profit margin for these smaller aircraft.
The larger Dreamliner is being used to hone manufacturing methods, then they can be economically applied to the smaller aircraft. Boeing would be stupid not to have this conversation now.
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.