Boeing's best selling product is the plain vanilla 737 single aisle jet liner. It's a good plane, it's been in production for a long time, it has orders that will take years to fill.
But, arch rival Airbus has announced a "New Engine Option" (NEO) for it's bread and butter airliner, the A320, direct competitor with Boeing's 737. Airbus will put Pratt & Whitney's new geared turbofan engine on the A320. Airbus publicity claims a 5% fuel savings. It is already beginning to gather orders, dispite the fact that it won't be delivered for years.
Question for Boeing. Should Boeing start design on a 737 replacement? Downside is enormous costs, embarrassing delays, an engineering department still tied up in knots with the long delayed 787 program. Plus the Boeing engineers can't come up with a plane that would be much better than the existing 737. Plus, as soon as a 737 replacement is announced, customers will delay orders, waiting for the new model to become available.
Driving Boeing toward a new design is the fear that the new Airbus plane will be decisively superior to the long-in-the-tooth 737 and capture the market. The 737 is the market, or at least the largest part of the market. Boeing sells ten 737's for every other model they sell.
As of now, Boeing hasn't said what they plan to do.