How do we really know this? The Japanese were ready for a long drawn out decades conflict. The US forces were war-weary and war was draining the economy. This could have gone on with quite some time, or ended in a frozen conflict that would reignite periodically. Who knows, but a quick non-nuclear win was certainly not in the cards.
>would the superpowers have been as restrained as they were in reality?
Arguably, if the US used nukes on Chinese soldiers during the Korean conflict, as some generals advised, there would be no North Korea. We'd have a cowed China and built a single free and democratic Korea, essentially one large South Korea. Instead we have one of the worst regimes the world has ever seen as a client state of China, who uses China's political influence and muscle to threaten the West with nuclear war every day. So, progress?
Conflicts need to happen. The more we put them off or try to find conditional surrenders or half-assed peace, the more we're asking for more trouble down the line.
imho, the cold war thinkers had no love for humanity and weren't influenced by the bombings in Japan. From a game theory perspective they had no incentive to fight a full scale war because it would annihilate both sides quickly. Its a losing outcome for all. Instead, it led to dozens of brutal proxy conflicts in non-nuclear states paid for by the lives of young men of those states. How many died because of the advance of Communism in Asia? How many states failed because of it? How many did Stalin send to their deaths?
These are not trivial numbers. You still had conflict by the USSR's expansionist agenda, and a serious amount of it, just with conventional arms while nuclear arms largely protected the West from a Soviet surprise nuke attack. Countries without nukes were gristle for the Soviet oppression machine.