I think that the 737 NG
series will need one update, well, you see that Airbus released the A320 family, and due to the caracteristics of this family of airliners like the system of gestion of containers, size, avionics, etc, well, the 737 NG
is just an evolution of the anterior series. Boeing says that is a completely new product, but it isn't, is just an evolution, the 737NG has a lot in common with the old series. Well, this is not a question of range, because the 737NG and the A320 have more or less the same range. I believe that boeing is going to launch a new generation of 737, with a system of cargo like the A320, bigger fusalage like the A320 family and new avionics.
the new plane will be Named 737 or 7X7 or other name... well a name is just a name; perhaps the 900 will be the last plane with this actual main characteristics, perhaps the new 737 will have a visual aspect quite diferent... and will use the same name... 737... it's like an Heritage eheheh
Cadillac started prodution of the Cadillac Deville many years ago... and the Caddy changed shape, model and design a lot of times... but it still the Cadillac DeVille.
Well with the 737 perhaps is more or less the same... perhaps Boeing wanted to preserve the identity and the brand 737 and produce an evolution or otherwise change for a new designation.
But true is... Boeing will replace current series... when market ask for it... they will!