I don't think this is a new phenomena. My understanding is that the MD-80 was built on the DC-9 airframe partially to avoid the costs and efforts of recertifica

I don't think this is a new phenomena. My understanding is that the MD-80 was built on the DC-9 airframe partially to avoid the costs and efforts of recertification, and that was 40 years ago. Aside from regulation, There are very good business reasons from both ends of the industry that exert pressure to iterate on an old design. (E.g Each additional airframe dramatically increases the complexity of an airline's maintenance organization.)