They dropped two nuclear bombs on Japan to make the Soviet Union's invasion of Hokkaido unnecessary and as a performative act to horrify the world in to subjugation.
The whole 'saved more lives than it cost' is a bad argument unless the plan was to slaughter a hundred thousand civilians on landing. Note the Nazi invasion of France cost 60,000 lives in civilian resistance...this was at the beginning of the war.
Japan would have surrendered on the first day of invasion.
The Japanese aren't a monolith, and that's one of the most irritating things trying to read about the Pacific campaign. It intrigues me as far as WW2 goes because the European campaign (in the west of Europe anyway) gets all the spotlight, with the Pacific campaign - especially the not-US contributions to it - gets a lot less attention. But unfortunately most of the writers are warhawks from the West who write about Japan as if it's this monolithic entity, as if every single person had the exact same BuShIdO sPiRiT and how the US couldn't invade because swarms of schoolchildren would attack them with grenades and sticks or something.
Meanwhile the Japanese are actual human beings and while bushido spirit type shit was taught to the officers and soldiers I highly doubt the claim that every single Japanese person was a secret ninja samurai ready to die for their country.
My Asian history professor was Japanese and he told us that his grandfather was arrested and thrown in prison for opposing the war and the emperor system. So yea, the idea that the Japanese were some sort of brainwashed horde that would fight to the death for their government is just racist, orientalist horseshit, there was opposition to the war and to Japanese imperialism within Japan.