The allies would have seen another 1.7 to 4 million killed.
Secretary of War Henry L. Stimson himself stated "We shall in my opinion have to go through an even more bitter finish fight than in Germany. We shall incur the losses incident to such a war and we shall leave the Japanese islands even more thoroughly destroyed than was the case with Germany."
“Up to ten million Japanese would have been killed if we invaded.”
This likely is true, however, with as much as the political class and every facet of government has lied to us (not just recent history) how can we take anything they say on face value? We should question everything.
A large part of United State objective during the war was to kill as many civilians (women, childern, babies, elderly) as possible to force a political outcome. That is no secret, thus the firebombing of tokyo and nuclear bombs. We did not nuke military targets.