Why do people say that an airplane that has been constantly improved over many years in service is still an old design? Lets take the 747-400 for example. The first 747 flew in 1970? But compare the 747-100 to the 747-400 and you'll see that they are two totally different airplanes. IMO, same concept, different design.
Same with the 737. To me 737-100 and 737-700/800/900 and 737 classics are totally different airplanes.
Same goes with the A340. A342/3 to me is different than the A345/6.
Anyone elses thoughts?