They will become part of the already existing 737-7/8 categories. That's one of the major reasons we ordered this instead of the Airbus, and why the 757 production line has stopped. Put simply, there are already too many 737's flying around the world to not build more. The pilots, mechanics, parts and simulators are already in place, so the cost of adding a "new" aircraft to the fleet becomes far less. Additionally, if a crew is flying in a 737-800 to say, Pittsburgh, they can fly the 737-900 out in the morning (as opposed to a 757) instead of having to wait for an 800 to come in. Makes for shorter layovers and more productivity from the pilots.

As a pilot though, it's disappointing to see 50 year old technology and design being used on what could have been a clean sheet design that incorporates the many technological advances we've made in half a century... all due to economics. I can't think of another industry that has been at the cutting edge of science and is now so hamstrung by governmental and corporate budgets. Think about it- most higher end cars have far more advanced technology on board than do our current batch of airliners.

The only thing I can imagine that will put the 737 out of production is an enormous leap forward in aerodynamic design, such as a flying wing, or materials technology that makes producing a 737 clone uneconomical. Neither of these possibilities are on the horizon now, and the 737 MAX makes them that much more remote.