Without Downfall and the nuclear program we're going to see economic strangulation of the Home Islands. By 1 or 2 years starvation and heavy bombardment by the Air Force will force them to give in, but Imperial hardliners might want to fight to the vety end without a sharp shock, so things could end up really badly
On the other hand, without a surrender the Red Army will just keep on pushing through Korea and might even attack Japan's armies in China. August Storm played a large part in the surrender IRL, so getting kicked out of China entirely plus the blockade might make a surrender likely quickly.