I wonder if this has to do with the overall decline of manufacturing in the US. Supply chains have been moving out of the US so iteration will be slower and more costly. Fewer talents would like to go to manufacturing. Fewer university professors are researching traditional mechanics now, for example, but more on "cutting-edge" stuff like nanometer materials. We also get fewer and fewer highly trained workers on lathing, welding, milling and etc over the years. As a result, it gets more expensive and more risky to build something as complex as F35.