We would have had to fight two wars in Japan, the first against Tojo, the second against the Communists.
Yep, as it was, we just postponed the war with the commies 5 years. But because of the decisive nature of the bomb, we had a base in Japan to fight the war in Korea, one not devastated by a blockade of an undetermined length, a period of time when if possible, the hatred between former enemies might have become more intractable. The success of a blockade would have been inevitable, but at what cost in lives, misery and expenditure of national treasure on both sides. Any speculation on its length is revisionism of the worst kind. The free world deserved a speedy end to the conflict, the bomb paved the way, no pun intended.