An evaluation of the advantages of these kind of certifications compared to not having updates would be interesting (do they really add value, except moving around responsibility?).
That would be a highly interesting evaluation. I worked in the Aerospace/Defense industry in the 1980s, and it seemed to me that "we can't change X, X is 'flight certified'" was a huge excuse for not innovating, or maybe a huge roadblock to innovation. So it's big news in 2016, when Boeing is hinting about stopping 747 production, an aircraft that made its first flight in February of 1969, 46 years before. I'm guessing that "flight certification" is the largest factor in keeping airliner technology in the 1960s.
At the same time, we had a good understanding about aerodynamics in the 1960s and were producing more or less optimized designs. There are some additional optimizations that we've figured out like sharklets, but overall the design is similar -- at least until we trusted composites enough to use them in aircraft.
Where we have seen a lot of innovation is in the engines -- fuel economy and noise regulations have pushed GE, RR and P&W to up their game substantially.
The 747 is only one example. Martin Marietta made and launched Titan space launch vehicles from the early 60s to the early 90s, with only very slight changes and improvements. GD did much the same with the Atlas launch vehicle, and the Centaur upper stage. I will grant that NASA and Douglas/McDonnel Douglas made a lot out of the Thor IRBM, but that seems like a function of NASA Administrators having longer tenure than anything else.
