Japan's wasted investment was trained people

What was Japan's worst investment in technology development during the war?

The greater waste was human capital: Japan spent years creating an elite carrier air arm and then failed to preserve its veterans, instructors, and institutional memory.

Yamato and Musashi invite debate over alternative uses of resources, but Japan's deeper waste was its failure to preserve the elite fleet air arm built during the 1920s and 1930s. Veterans were not withdrawn in sufficient numbers to form cadres for new units and training schools, so combat losses erased experience faster than aircraft production could replace capability. The problem was less invention than turning technology into hardware operated by skilled people.