You're misunderstanding the purpose of nuclear bombs, particularly against largely wooden cities such as Japan had at the time. Large scale firebombing against targets with negligible AAA are more destructive than individual early nukes. However, that's not the point. Nuclear weapons are golden beebees. Targets with heavy AAA (like the Soviets, *hint hint*) can cause 50% attrition on bomber raids and reduce the damage by 50% or more. Likely more since AAA prevented precision bombing of the era. Against golden beebees, it doesn't matter if you shoot down 50% of the bombers if any single one can carry a nuke that mission kills your city. It's like preventing pregnancy. You either need to stop 100% of the sperm or you're going to lose. The USA used nukes to accelerate the war with Japan, but primarily as a show of force against the USSR.