My Dad who was in Hiroshima 6 weeks after the bomb told me if they had had to invade those fortified beaches the US would have lost at least 500,000 men.
By the time the war ended, the Japanese actually possessed some 12,700 aircraft in the Home Islands, roughly half kamikazes...By the time of surrender, the Japanese had over 735,000 military personnel either in position or in various stages of deployment on Kyushu alone. The total strength of the Japanese military in the Home Islands amounted to 4,335,500, of whom 2,372,700 were in the Army and 1,962,800 in the Navy
https://en.wikipedia.org/wiki/Operation_Downfall#Air_threat