Originally Posted by
tdracer
Minor point, the NG wing is not 'new' - it was aerodynamically re-profiled (using the same structure as the classic). The new aero bought a little drag but it also bought speed - the NG critical Mach was meaningfully higher than the classic - which given the NG's longer legs was meaningful. Some that had worked on the 737-300 complained that they'd wanted to make similar changes on the -300 but were not allowed to.
The thing that really killed going to an all new 737 replacement was how long it would take to bring it on-line. Resources were still tight with the 787 and 747-8 just entering service, and not only would it have taken a couple years longer to develop than the MAX, it would have taken another few years to bring the production rate up to the 50/month ballpark. That would have meant conceding something like 3000 narrow body sales to the NEO - something that Boeing saw as untenable (although with the benefit of 20-20 hindsight doesn't look so bad).
A minor point on your minor point....
the NG forward section removed the drooped cuff that was placed on the Classic wing profile of the original section. The weight increase of the Classic over the 200 resulted in a need for higher Cl/aoa, gained by the cuff. The NG extended the wing span, removed the cuff, and added a re-profiling of the aft section of the mid and inboard wing providing mild aft loading. The ailerons were increased in span, and reduced in chord which permitted removal of the outboard upper surface VGs, the shockwave remains clear of the hinge line at high speed. Flap system was simplified, and track design removed the residual tracks that the Classic carried over from the JT8 aft nacelle. The thrust gate design of the Classic flap was cleaned up, giving an improvement in flap effect, while mainly avoiding impingement on the flap and associated fatigue issues.
The Classic ran out of goodness around .745 but could be pushed higher if needed at considerable fuel burn. The NG for the same weight is running .785-.795 on lower fuel burns than the classic. To run a Classic at .79 takes a lot of extra gas... and while well within the operating boundary is associated with noticeable buffet from the aileron/shock separation. The NG has relatively high approach speeds, which is a tradeoff of the simplification of the flaps. Randolph's paper on high lift devices covers the tradeoffs well.
The Classic EFIS functioned, but the PFD/ND/EIS is supportable and upgradeable on the NG, the Classic being difficult to upgrade piecemeal. The Classic handles better IMHO, but the NG is a more effective transport aircraft,
As far as the fuselage goes, the double bubble profile precludes underfloor LD3 loading, Airbus picks up freight opportunities from any 737 operators.