The following is premised on:
the invasion still takes place at all (Nimitz and King's opposition doesn't kill it), Olympic goes ahead more or less as planned if it does (target isn't changed to Shikoku or elsewhere),
Soviet plans get carried out
The invasion would be a minor disaster. The Nimitz and King vs MacArthur split in command over the invasion doesn't bode well. (Not saying that any of them would intentionally tank it, just that ordering a commander to carry out an invasion they don't fully believe in usually has less than optimal results.) The US casualty rates won't be as high as many if the ridiculous estimations, but will be higher than otherwise. Japanese casualties will be catastrophic - quite easily 20% or more of the entire population (factoring in famine and concurrent diseases) of the total population. I'd say
Kyushu will be a Iwo and Okinawan meat grinder. Instead of Hiroshima and Nagasaki, every American would know the city of Kagoshima as the Japanese city devastated at the end of the war, more a Manila writ large - almost (but not quite) a Stalingrad.
The rather optimistic (time frame wise) plans for Coronet would be drastically changed. Hard to say exactly how the rest of the invasion would go down, but sometime in the early spring, Japan functionally ceases to exist due to a combination of a failed rice crop, bombing, blockade, and devastated infrastructure.
Occupation - such as it is - will be totally different. The emperor may very well be tried as a class A. (If so, conviction is unlikely.) It won't be nice like SCAP. I hope the US sends food relief, but I expect it'll be a variation of Morgenthau.
The shape of post-war Asia will likewise be different. The Soviets will get all of Korea and probably Hokkaido. Mao and the PLA will get a big boost.
People argue back and forth about the atomic bombings or the Soviet DoW. I don't think you can separate the three events.
Indeed.