Actually, no one had superfortresses. The only real NA ever used was Russian Winter and Fortress Europe…the rest were forgotten since no one ever played with them.
Meanwhile, if you can scare Japan into SZ 61 or destroy him, you have effectively contained the Jap fleet. IF you can tkae SZ 60 even if Japan isn’t there, and hold it, you have contained Japan. (At least you’ve removed their largest production facility.)
Japan is very flimsy. Ask anyone who’s worked on a KJF strat. One or two bad moves with Japan and Japan is neutrallized. One or 20 bad moves with America and at worst Russia has fallen.