When the hell was a 737 ever called a "pocket rocket" ???? Only thing the classics are faster than is the 146! The NG's are "pocket rockets" by 737 standards and a very nice airplane to boot.
Back to the original question though. Keeping an airplane looking the same over the years does save a lot on costs. If Boeing had put all the nice 777 gear in a "classic" 737 it would indeed have been at great cost for no appreciable change in aircraft economics. Ultimately they did the right thing and redesign the wings and use better engines as well as incorporate the latest avionics technology. Now the cost was worth the effort. The overhead had remained mostly unchanged as I understand it because the FAA would not allow the common type rating if the modernised the swichology to any great extent. That is it would have been one to many improvements and a separate type rating would have been required.