I have to agree with Boeing nut. The wing are new. The engines are new. The cockpit is entirely new. The cabin is new. The systems (electrical, hydraulic) are new. So... what else is left? The shell. The fuselage. It's a time- proven airframe, known for it's inherent strength and durability.
Incidentally, you compare it to the Mustang. While that doesn't offend me at all, I think it would now be appropriate to point out that the Ford Mustang is the BEST SELLING CAR IN
ITS CLASS. It shoved the Firebird and Camaro out of production last year- and it has been around longer (Mustang in 1964, Camaro came out of the stable in 1967).
There are airlines that are making profits flying 737s. Southwest. There are airlines that are profitable flying A320s. JetBlue.
I don't think it warrants the billions of dollars needed to R/D, build, test, et cetera, an entirely new airframe. The 777 was a much needed airplane in Boeing's lineup. A new 737 is not. It remains one of the best selling airframes, currently. And I blame the A320s recent sales success NOT on winning a large number of customers, but on winning large orders. I mean, Easyjet or whoever it was ordered a huge number. JetBlue keeps ordering 40 at a time. But Boeing successfully pitched the 737 to AirTran (which surprised me, I feared that they would go 'Bus for sure).
The design of the wheel goes back millennia. It will never get any rounder. It may get lighter. It may get better bearings. But it's still just a wheel.