At "best" they would have been able to take a port in northern Hokkaido, which would have been terribly obnoxious after the war, but not by itself strategically significant after they'd taken so many of the islands to the north. Details about this, and the massive resources that would have been required by us to take just Sapporo, can be found in the highly recommended Hell to Pay: Operation Downfall and the Invasion of Japan, 1945-1947 (https://www.amazon.com/Hell-Pay-Operation-Downfall-1945-1947...) I recommend elsewhere in this subthread.

To imagine this was a major factor in the decision, when e.g. Truman's ship to the Potsdam Conference crossed the path of 4 US divisions being shuttled from the ETO to the Pacific is ludicrous. E.g. we're still issuing Purple Hearts from the initial lot MacArthur ordered, which was estimated to be 1/4 of what would be needed for Operation Downfall. This was also not long after the survivors of the Bataan Death March had been rescued, we were very much in a mood of revenge, see Grove's autobiography for a direct citation of this in reference to the bombing of Hiroshima.