The goal wasn’t to hold the islands after the IJN was defeated; it was to extract so heavy a toll that the Americans would not dare try to invade mainland Japan. If you believe that the IJA would not send many more thousands of soldiers (if it could) to die on those islands to extract a higher amount of American blood, then you are gravely mistaken.
I believe they would i simply do not believe that they could have done so. One to find the ships or planes or worse submarines to get them there. they first have to get ships from a shrinking merchant marine and navy. have them survive long enough to get to the island, then have to supply the island so they don’t starve or run out of shells. numbers mean a lot but not everything if the garrisons are taking lots for who goes in the cooking pot.