Japan wouldn't have lasted until April of 1947 (24 months after FDR died). Even without the Bomb, even if the U.S. and/or the USSR invades the Home Islands, Japan was never going to survive that long.
By the end of 1946, simply if the U.S. continued with the submarine and air campaigns, Japan would have been quite literally, have been bombed back to the 15th Century and the only way the population would be surviving was through organized canibalism. The U.S. was ready to stand up the 8th Air Force on Okinawa, which would have enabled nightly firebombing of every population center in the entire Home Islands, including Hokkiado (which was out of range for Tinian based bombers). When one considers that the USAAF had been forced to designate 20 cities as untouchable to ensure they actually had a decent set of targets to use the Bomb against before the 8th was operational, the addition of an entire extra Air Force (1,000+ bombers x2 fighters) almost 100% of which were crewed by veterans of the ETO almost boggles the mind.
Throw in the fact that the USSR would, at minimum, have utterly obliterated the IJA on Northern Asian Mainland (it it reasonable to assume that the Red Army would not have made it all the way to the Gulf of Tonkin and would have met Allied forces somewhere in central China) ensureing that absolutely no food reached the Home Islands from Asai, even on small coastal vessels too small for reasonable submarine interception, and that American fighter bombers would have been doing exactly what they did the last two months of the war in Europe, namely attacking anything with wheels, right down to horse and goat drawn carts, and you have a population and countryside starved and burned out to a point that hasn't been seen since the Mongols went out of the Horde business.