I think you've learned the wrong lesson from the F35's development. It likely would've been more cost efficient to build three different planes that are special

I think you've learned the wrong lesson from the F35's development. It likely would've been more cost efficient to build three different planes that are specialized to the needs of each branch of the military. One reason that the plane has been such a boondoggle (1.5 trillion in development cost) is that it is trying to be everything for everyone. Optimizing a fighter jet for replacement parts is monumentally stupid. It should be optimized for its ability to perform its job in the field and it turns out each branch needs a different type of plane. Stockpiling several sets of parts is of least concern and is unlikely to cost more money.