I&#x27;m not an airplane engineer, but I think a fair amount of it is technical debt, not the quality of the DC-9 design. For example, the 737 - a competing design that came out around the same time - is not only still being made, it&#x27;s is getting prepped for another minor revision (the 737 Max), and the newest narrowbody airliner design in common worldwide usage (the A320 series) is going on 30 years old. The 717 was launched as the MD-95, a project at McDonnell Douglas to update the DC-9 series for the 90s, and Boeing (after merging with McDonnell Douglas) mostly just built out the already-sold MD order book and then shut the line down.<p>It would likely be very good for a manufacturer to make a new, clean-sheet design with updates like a carbon fiber fuselage, but the time needed to bring such a product to market would cause major problems for any company attempting such a project.