Only two I-400s and a small number of adapted aircraft-carrying submarines were available, leaving no way to keep constant pressure on an American coast. They also arrived after Japan had lost much of its carrier strength, naval aviation, and wider fleet, leaving little force that an Allied diversion could help in a decisive battle. By 1944-45 the United States and its allies were producing escorts, maritime patrol aircraft, radar, and other defenses at overwhelming scale. The same kind of threat that might have pulled destroyers away from Coral Sea or Midway would be far easier to absorb late in the war. Expanding an established production run by a fraction was also much easier than creating a large new defensive network from the limited output available in 1941.
Notable Point · 42:44-46:23
Two boats could not create strategic pressure
By the time the I-400s entered service, too few existed to sustain the raids that might have forced a major Allied diversion.